This PR adds additional background and border colors for each of the
status colors to the theme.

These colors are then used to style the diagnostic popovers:

<img width="407" alt="Screenshot 2024-01-02 at 4 25 50 PM"
src="https://github.com/zed-industries/zed/assets/1486634/fbc64ea7-3dcd-43b7-8520-91f16304bfba">

<img width="396" alt="Screenshot 2024-01-02 at 4 26 05 PM"
src="https://github.com/zed-industries/zed/assets/1486634/6e4a44bf-4aac-4595-afff-7fbf8b2b9e1f">

Release Notes:

- N/A

This PR fixes the spacing in the diagnostics tab.

#### Before

<img width="108" alt="Screenshot 2024-01-02 at 3 40 51 PM"
src="https://github.com/zed-industries/zed/assets/1486634/9508d867-5b5d-45b0-8b16-be35a23e3e7a">

#### After

<img width="121" alt="Screenshot 2024-01-02 at 3 40 54 PM"
src="https://github.com/zed-industries/zed/assets/1486634/33e665e7-2935-4d4f-b1d0-9cb655235867">

Release Notes:

- N/A

This PR restores the semi-transparency to the scroll bar handle colors
that were lost in #3842.

If we adjust the rendering of the scrollbar to draw the status
indicators on top of the scrollbar handle then we can remove the
transparency.

Release Notes:

- N/A

Bring Zed1 themes closer to their old appearance using the `base_theme`
values.

This will inevitably re-introduce some outstanding design issues from
Zed 1 that we will need to solve (low contrasts in some places, etc.)
We'll work on some of these in a later PR.

Known issues introduced:
- Scrollbar thumb is incorrectly drawn in front of the indicators
- Borders aren't quite correct - will fix in a followup
- Titlebar/status bar buttons will now have low contrast states
- Keybinding background color blends with palette

There are likely more issues, add them to the product checklist as you
find them.

Release Notes:

- N/A

Co-Authored-By: Marshall Bowers <1486634+maxdeviant@users.noreply.github.com>

Note: We removed some unused theme colors
Co-Authored-By: Marshall Bowers <1486634+maxdeviant@users.noreply.github.com>

Co-Authored-By: Marshall Bowers <1486634+maxdeviant@users.noreply.github.com>

Co-Authored-By: Marshall Bowers <1486634+maxdeviant@users.noreply.github.com>

This PR absolutely positions the channel buttons on top of the channels.

This prevents the buttons from getting pushed off the edge of the panel
when the channel names are long.

Still needs some fine-tuning, but gets us closer to where we want to be.

Release Notes:

- N/A

It's not quite complete, but we gotta start somewhere.
Release Notes:

- N/A

Previously, we changed borders to be drawn after content, so they are no
longer part of the same quads as the background. In our change, we gave
the background quad a transparent black border and the border quads
transparent black backgrounds. However, this caused the other channels
to blend toward that black color before becoming fully transparent,
causing them to become darker.

In this PR, I source the "placeholder" color by duplicating the values
for the other channels and only adjust the alpha down to zero.

Release Notes:

- N/A

This makes it a bit more approachable to implement.

This required converting the state of Component to AnyElement, which
costs an allocation. I'm hoping this will be ok performance-wise now
that AnyElements use bump allocation, but I need to benchmark still.

This PR maps the suggestions colors from the Zed1 themes to the Zed2
themes.

Release Notes:

- N/A

This PR adds general-purpose functions for obtaining a `Length` in
viewport units.

Previously in #3600 we had added specific variants of `w` and `h` that
took viewport units, but I think it makes more sense to just have a
primitive that can compose with any styling method that accepts a
`Length` or `Into<Length>`.

Release Notes:

- N/A

This PR adjusts the size of the chat and channel note icons in the
channel list to be smaller.

Release Notes:

- N/A

This PR increases the max height of the outline picker so that it can
take up a larger area of the screen when there are lots of results.

This behavior is similar to the way it was in Zed1.

Release Notes:

- N/A

This PR maps the drop target background color in the `theme_importer`.

Release Notes:

- N/A

The word "new" is shorter and blends in with `new` constructors that are
common in Rust. Been meaning to do this for a while.

Release Notes:

- N/A

Fixes:
Project search does not have breadcrumbs on its first open: after switching to other tab and back, breadcrumbs are shown

We still have a bunch of leftovers in ui2/theme2 and in gpui2
display_link module.

Release Notes:

- N/A

This commit also changes the way search queries are built (we do not bail early anymore if include/exclude editor queries are malformed) to propagate error status of the panel.
Release Notes:

- N/A

The word "new" is shorter and blends in with `new` constructors that are common
in Rust. Been meaning to do this for a while.

Getting some scaffold docs in place and making some naming adjustments
to improve consistency with Rust APIs and clarity.

- Render::Element -> Render::Output, and Render::Output is now only
required to be IntoElement instead of Element so we can return
components.
- RenderOnce::Element -> RenderOnce::Output
- Element::layout -> Render::request_layout - Clarify that this doesn't
actually *perform* layout, but simply requests

Release Notes:

- N/A