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/extensions_ui/src/components/extension_card.rs

@@ -0,0 +1,40 @@
+use gpui::{prelude::*, AnyElement};
+use smallvec::SmallVec;
+use ui::prelude::*;
+
+#[derive(IntoElement)]
+pub struct ExtensionCard {
+    children: SmallVec<[AnyElement; 2]>,
+}
+
+impl ExtensionCard {
+    pub fn new() -> Self {
+        Self {
+            children: SmallVec::new(),
+        }
+    }
+}
+
+impl ParentElement for ExtensionCard {
+    fn extend(&mut self, elements: impl Iterator<Item = AnyElement>) {
+        self.children.extend(elements)
+    }
+}
+
+impl RenderOnce for ExtensionCard {
+    fn render(self, cx: &mut WindowContext) -> impl IntoElement {
+        div().w_full().child(
+            v_flex()
+                .w_full()
+                .h(rems(7.))
+                .p_3()
+                .mt_4()
+                .gap_2()
+                .bg(cx.theme().colors().elevated_surface_background)
+                .border_1()
+                .border_color(cx.theme().colors().border)
+                .rounded_md()
+                .children(self.children),
+        )
+    }
+}