chore: Extract assets module out of zed crate. (#3957)

This essentially shaves off about 10% off of an incremental build after
project change and potentially more if you're changing stuff like
`welcome` that's very close to the `zed` crate in the dep graph. That's
because macro expansion takes place even in incremental builds it seems?
And zed (lib) + zed (bin) could take up to 4 seconds out of an
incremental build, which is a *lot* in a 10s build. In reality though it
shaves 1 second off of 5 seconds incremental 'welcome'/ 1s off of 10s
'project' builds.

Note that we had `assets` crate in the past (removed in #2575 /cc
@maxbrunsfeld), but this is a bit different, because `assets` is a
dependency of *just* zed and nothing else. We essentially cache macro
expansion results ourselves.

Release Notes:

- N/A

crates/zed/Cargo.toml

@@ -74,6 +74,7 @@ vim = { path = "../vim" }
 workspace = { path = "../workspace" }
 welcome = { path = "../welcome" }
 zed_actions = {path = "../zed_actions"}
+assets = {path = "../assets"}
 anyhow.workspace = true
 async-compression.workspace = true
 async-tar = "0.4.2"