Speed up cargo build -p zed compilation (#62059)

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 (0500fd9ee9): 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

.github/workflows/run_tests.yml

@@ -708,6 +708,8 @@ jobs:
       run: cargo machete
     - name: run_tests::check_dependencies::check_cargo_lock
       run: cargo update --locked --workspace
+    - name: run_tests::check_dependencies::check_crate_graph
+      run: 'cargo test --package xtask -- workspace::'
     - name: run_tests::check_dependencies::check_vulnerable_dependencies
       if: github.event_name == 'pull_request'
       uses: actions/dependency-review-action@67d4f4bd7a9b17a0db54d2a7519187c65e339de8