Extract PureScript support into an extension (#9824)

This PR extracts PureScript support into an extension and removes the
built-in PureScript support from Zed.

Release Notes:

- Removed built-in support for PureScript, in favor of making it
available as an extension. The PureScript extension will be suggested
for download when you open a `.purs` file.

extensions/purescript/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "zed_purescript"
+version = "0.0.1"
+edition = "2021"
+publish = false
+license = "Apache-2.0"
+
+[lints]
+workspace = true
+
+[lib]
+path = "src/purescript.rs"
+crate-type = ["cdylib"]
+
+[dependencies]
+zed_extension_api = "0.0.4"