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/assets/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "assets"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+gpui = {path = "../gpui"}
+rust-embed.workspace = true
+anyhow.workspace = true

crates/assets/src/lib.rs

@@ -0,0 +1,36 @@
+// This crate was essentially pulled out verbatim from main `zed` crate to avoid having to run RustEmbed macro whenever zed has to be rebuilt. It saves a second or two on an incremental build.
+use anyhow::anyhow;
+
+use gpui::{AssetSource, Result, SharedString};
+use rust_embed::RustEmbed;
+
+#[derive(RustEmbed)]
+#[folder = "../../assets"]
+#[include = "fonts/**/*"]
+#[include = "icons/**/*"]
+#[include = "themes/**/*"]
+#[exclude = "themes/src/*"]
+#[include = "sounds/**/*"]
+#[include = "*.md"]
+#[exclude = "*.DS_Store"]
+pub struct Assets;
+
+impl AssetSource for Assets {
+    fn load(&self, path: &str) -> Result<std::borrow::Cow<[u8]>> {
+        Self::get(path)
+            .map(|f| f.data)
+            .ok_or_else(|| anyhow!("could not find asset at path \"{}\"", path))
+    }
+
+    fn list(&self, path: &str) -> Result<Vec<SharedString>> {
+        Ok(Self::iter()
+            .filter_map(|p| {
+                if p.starts_with(path) {
+                    Some(p.into())
+                } else {
+                    None
+                }
+            })
+            .collect())
+    }
+}