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

Cargo.lock

@@ -292,6 +292,15 @@ version = "1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d92bec98840b8f03a5ff5413de5293bfcd8bf96467cf5452609f939ec6f5de16"
 
+[[package]]
+name = "assets"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "gpui",
+ "rust-embed",
+]
+
 [[package]]
 name = "assistant"
 version = "0.1.0"
@@ -9529,6 +9538,7 @@ dependencies = [
  "activity_indicator",
  "ai",
  "anyhow",
+ "assets",
  "assistant",
  "async-compression",
  "async-recursion 0.3.2",