Splits the crate graph.
Before, the compilation graph: `editor → picker_preview → search →
project_panel → open_path_prompt → recent_projects → title_bar →
collab_ui → zed`
After, the compilation graph: `editor → picker_preview → search →
agent_ui → sidebar → zed`

With sccache disabled and project fully built, `touch
crates/editor/src/editor.rs` and `cargo build -p zed` took time
Before (5e1fd392f6): 13.19s
After: 11.85s (-10.2% speed up)

Each commit contains a separate compilation instructions, and a test in
the final commit.
The main approach is to split coupled crates and replace them with
`zed_actions` and move some shared functionality into `git_ui_core` new,
shared module.

I've also tried to add a test to prevent common pitfalls, but not sure
I'm happy with the end result — can remove it if it looks too synthetic.

Release Notes:

- N/A