Eliminate assets crate (#2575)

Previously, all static assets were embedded into Zed using a single
`RustEmbed` struct called `Assets`, which was in a crate of its own,
also called `assets`. Many crates depended on this crate. The problem
with this situation was that changing *any* static asset file caused
almost every crate in the codebase to need recompilation (because of
recursive dependencies on `assets`).

Now, most of the assets are embedded only into the top-level `zed`
crate. A few assets (such as settings JSON files and AI prompts) are
needed in lower-level crates such as `settings` and `ai`. For these,
I've created separate `RustEmbed` structs in those specific crates,
which embed those specific sub-folders of `assets`.

The result is that now, when you change a theme file, the only crate
that needs to recompile is `zed`.

Release Notes:

- N/A