Implement a more robust way of locating rust-analyzer

When bundled, we will retrieve it out of the `Resources` folder.
Locally, we're expected to run `script/download-rust-analyzer` and
put `vendor/bin` in our $PATH.

crates/lsp/Cargo.toml

@@ -13,3 +13,6 @@ parking_lot = "0.11"
 serde = { version = "1.0", features = ["derive"] }
 serde_json = { version = "1.0", features = ["raw_value"] }
 smol = "1.2"
+
+[dev-dependencies]
+gpui = { path = "../gpui", features = ["test-support"] }