This PR factors out the bulk of the boilerplate required to setup a
story in the storybook out into separate components.

The pattern we're using here is adapted from the "[associated
component](https://maxdeviant.com/posts/2021/react-associated-components/)"
pattern in React.

Release Notes:

- N/A

Reorganized assistant/semantic_index crates and introduced AI crate to
include shared functionality.

Release Notes:

- Moved most of the Assistant functionality from ai crate to assistant
crate
- Moved interaction with embedding providers from semantic_index to ai
crate

Last follow-up of https://github.com/zed-industries/zed/pull/3002
Fixes
https://zed-industries.slack.com/archives/C04S6T1T7TQ/p1695281196667609

Copilot sends multiple events about its LSP server readiness, not
necessarily recreating the server from scratch (e.g. due to re-sign in
action). Avoid re-adding same log subscriptions on the same LSP server,
which causes panics.

Release Notes:

- N/A

Copilot sends multiple events about its LSP server readiness, not necessarily recreating the server from scratch (e.g. due to re-sign in action).
Avoid re-adding same log subscriptions on the same LSP server, which
causes panics.

Merges various in-progress gpui2 component branches with the new `ui`
crate.

This PR adds a `TrafficLights` component for GPUI2.

<img width="861" alt="Screenshot 2023-09-21 at 11 32 10 PM"
src="https://github.com/zed-industries/zed/assets/1486634/0fe0e847-49b3-44dc-bd4c-64f12f0051c1">

Release Notes:

- N/A

This PR updates the `storybook` with support for adding stories for
individual components.

### Motivation

Right now we just have one story in the storybook that renders an entire
`WorkspaceElement`.

While iterating on the various UI components, it will be helpful to be
able to create stories of those components just by themselves.

This is especially true for components that have a number of different
states, as we can render the components in all of the various states in
a single layout.

### Explanation

We achieve this by adding a simple CLI to the storybook.

The `storybook` binary now accepts an optional `[STORY]` parameter that
can be used to indicate which story should be loaded. If this parameter
is not provided, it will load the workspace story as it currently does.

Passing a story name will load the corresponding story, if it exists.

For example:

```
cargo run -- elements/avatar
```

<img width="723" alt="Screenshot 2023-09-21 at 10 29 52 PM"
src="https://github.com/zed-industries/zed/assets/1486634/5df489ed-8607-4024-9c19-c5f4541f97c9">

```
cargo run -- components/facepile
```

<img width="785" alt="Screenshot 2023-09-21 at 10 30 07 PM"
src="https://github.com/zed-industries/zed/assets/1486634/e04a4577-7403-405d-b23c-e765b7a06229">



Release Notes:

- N/A

Fixes an ambiguous reference to `theme` causing storybook not to build.

This PR extracts the various UI elements from the `storybook` crate into
a new `ui` library crate.

Release Notes:

- N/A

This PR reorganizes the exports for the `ui` module in the `storybook`
crate.

### Motivation

Currently we expose each of the various elements/components/modules in
two places:

- Through the module itself (e.g., `ui::element::Avatar`)
- Through the `ui` module's re-exports (e.g., `ui::Avatar`)

This means it's possible to import any given item from two spots, which
can lead to inconsistencies in the consumers. Additionally, it also
means we're shipping the exact module structure underneath `ui` as part
of the public API.

### Explanation

To avoid this, we can avoid exposing each of the individual modules
underneath `ui::{element, component, module}` and instead export just
the module contents themselves.

This makes the `ui` module namespace flat.

Release Notes:

- N/A

Update project search semantic ui to accommodate for users who have not
set the OPENAI_API_KEY in their environment variables.

Release Notes:

- Expand Semantic Index status to include status for non authenticated
users
- Update Search UI to illustrate this status.

Fixes a design regression on Preview where the fold icon became small
due to the icon standardization PR.

Release Notes:

- [Preview] Fixed an issue with the size of the fold line icon.

Release Notes:

- Fixes a bug where channels could no longer be rearranged with drag and
drop.

Targeting Preview of 09.27.
This is still pending several touchups/clearups:
- We should watch multibuffer for changes and rescan the excerpts. This
should also update match count.
- Closing editor while multibuffer with 100's of changed files is open
leads to us prompting for save once per each file in the multibuffer.
One could in theory save in multibuffer before closing it (thus avoiding
unnecessary prompts), but it'd be cool to be able to "Save all"/"Discard
All".

Release Notes:

- Added "Replace in project" functionality

Follow-up of https://github.com/zed-industries/zed/pull/2991 improving
rough edges around supplementary LSP servers:

* Fixes
https://zed-industries.slack.com/archives/C04S6T1T7TQ/p1695281196667609
Copilot init panic
* Makes LSP server list scrollable in the panel
* Shows supplementary servers' RPC logs in the panel

Release Notes:

- N/A

co-authored-by: Max <max@zed.dev>