Fix collaboration regressions

Enhance language::tests::test_random_collaborators so that it checks buffer invariants.

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

A false negative return value of `gt` was preventing guests' multibuffers from
syncing correctly.

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

Also fix false failure in ModelHandle::condition when parking is not forbidden.

Fix buffer replication bugs that would lead to divergence among replicas

This test will exercise serialization of operations as well as peers
replicating from an existing buffer.

The main reason for this is that we need to include information about
a buffer's UndoMap into its protobuf representation. But it's a bit
complex to correctly incorporate this information into the current
protobuf representation.

If we want to continue reusing `Buffer::apply_remote_edit` for
incorporating the historical operations, we need to either make
that method capable of incorporating already-undone edits, or
serialize the UndoMap into undo *operations*, so that we can apply
these undo operations after the fact when deserializing. But this is
not trivial, because an UndoOperation requires information about
the full offset ranges that were undone.

Co-Authored-By: Antonio Scandurra <me@as-cii.com>

Co-Authored-By: Max Brunsfeld <max@zed.dev>

Miscellaneous bug fixes after switching to a project-centric sharing model

Co-Authored-By: Max Brunsfeld <max@zed.dev>

Use downcasting instead for accessing worktree-specific state of the Files.

This will allow us to introduce a WorktreeId type and use that everywhere
for identifying worktrees. It also just removes some unnecessary coupling
between the language crate and the worktree.

Allow the zed app to connect to both the old and new rpc endpoints

In the case of the new Next.js app, the app will follow a redirect
from 'zed.dev/rpc' to the subdomain where the rust service is hosted.
Until then, the app will connect directly to zed.dev/rpc.

Support moving lines up and down in multi-buffers

This allows us to respect the bias on anchors we use to create excerpt headers so that they always remain above any content inserted at the start of an excerpt.

Fix panics when moving lines with block decorations and simplify line boundary detection

Because multi-buffers can contain the same content multiple times, we need to use anchors to track our desired insertion and removal locations when moving lines. This is because deleting a line in order to move it might end up deleting *multiple* lines.

Maybe we should fail more gracefully in this case, but I think we should at least make the message accurate and see how we do.

Since these methods take buffer points instead of display points, this adjusts
the logic for retrieving the visible selections, so that they are initially returned
in terms of buffer points.

Use this method to disable move_line_down across excerpt boundaries.

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

This is still a bit weird because we can't remove the last line of an excerpt but we still move it into another buffer. There also seem to be issues with undo.