Add a command for building and installing a locally-developed Zed extension (#8781)

This PR adds an `zed: Install Local Extension` action, which lets you
select a path to a folder containing a Zed extension, and install that .
When you select a directory, the extension will be compiled (both the
Tree-sitter grammars and the Rust code for the extension itself) and
installed as a Zed extension, using a symlink.

### Details

A few dependencies are needed to build an extension:
* The Rust `wasm32-wasi` target. This is automatically installed if
needed via `rustup`.
* A wasi-preview1 adapter WASM module, for building WASM components with
Rust. This is automatically downloaded if needed from a `wasmtime`
GitHub release
* For building Tree-sitter parsers, a distribution of `wasi-sdk`. This
is automatically downloaded if needed from a `wasi-sdk` GitHub release.

The downloaded artifacts are cached in a support directory called
`Zed/extensions/build`.

### Tasks

UX

* [x] Show local extensions in the Extensions view
* [x] Provide a button for recompiling a linked extension
* [x] Make this action discoverable by adding a button for it on the
Extensions view
* [ ] Surface errors (don't just write them to the Zed log)

Packaging

* [ ] Create a separate executable that performs the extension
compilation. We'll switch the packaging system in our
[extensions](https://github.com/zed-industries/extensions) repo to use
this binary, so that there is one canonical definition of how to
build/package an extensions.

### Release Notes:

- N/A

---------

Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/extension/src/extension_manifest.rs

@@ -0,0 +1,72 @@
+use collections::BTreeMap;
+use language::LanguageServerName;
+use serde::{Deserialize, Serialize};
+use std::{path::PathBuf, sync::Arc};
+
+/// This is the old version of the extension manifest, from when it was `extension.json`.
+#[derive(Clone, PartialEq, Eq, Debug, Deserialize, Serialize)]
+pub struct OldExtensionManifest {
+    pub name: String,
+    pub version: Arc<str>,
+
+    #[serde(default)]
+    pub description: Option<String>,
+    #[serde(default)]
+    pub repository: Option<String>,
+    #[serde(default)]
+    pub authors: Vec<String>,
+
+    #[serde(default)]
+    pub themes: BTreeMap<Arc<str>, PathBuf>,
+    #[serde(default)]
+    pub languages: BTreeMap<Arc<str>, PathBuf>,
+    #[serde(default)]
+    pub grammars: BTreeMap<Arc<str>, PathBuf>,
+}
+
+#[derive(Clone, PartialEq, Eq, Debug, Deserialize, Serialize)]
+pub struct ExtensionManifest {
+    pub id: Arc<str>,
+    pub name: String,
+    pub version: Arc<str>,
+
+    #[serde(default)]
+    pub description: Option<String>,
+    #[serde(default)]
+    pub repository: Option<String>,
+    #[serde(default)]
+    pub authors: Vec<String>,
+    #[serde(default)]
+    pub lib: LibManifestEntry,
+
+    #[serde(default)]
+    pub themes: Vec<PathBuf>,
+    #[serde(default)]
+    pub languages: Vec<PathBuf>,
+    #[serde(default)]
+    pub grammars: BTreeMap<Arc<str>, GrammarManifestEntry>,
+    #[serde(default)]
+    pub language_servers: BTreeMap<LanguageServerName, LanguageServerManifestEntry>,
+}
+
+#[derive(Clone, Default, PartialEq, Eq, Debug, Deserialize, Serialize)]
+pub struct LibManifestEntry {
+    pub kind: Option<ExtensionLibraryKind>,
+}
+
+#[derive(Clone, PartialEq, Eq, Debug, Deserialize, Serialize)]
+pub enum ExtensionLibraryKind {
+    Rust,
+}
+
+#[derive(Clone, Default, PartialEq, Eq, Debug, Deserialize, Serialize)]
+pub struct GrammarManifestEntry {
+    pub repository: String,
+    #[serde(alias = "commit")]
+    pub rev: String,
+}
+
+#[derive(Clone, PartialEq, Eq, Debug, Deserialize, Serialize)]
+pub struct LanguageServerManifestEntry {
+    pub language: Arc<str>,
+}