Add a schema to extensions, to prevent installing extensions on too old of a Zed version (#9599)

Release Notes:

- N/A

---------

Co-authored-by: Marshall <marshall@zed.dev>

crates/extension_api/README.md

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+# The Zed Rust Extension API
+
+This crate lets you write extensions for Zed in Rust.
+
+## Extension Manifest
+
+You'll need an `extension.toml` file at the root of your extension directory, with the following structure:
+
+```toml
+id = "my-extension"
+name = "My Extension"
+description = "..."
+version = "0.0.1"
+schema_version = 1
+authors = ["Your Name <you@example.com>"]
+repository = "https://github.com/your/extension-repository"
+```
+
+## Cargo metadata
+
+Zed extensions are packaged as WebAssembly files. In your Cargo.toml, you'll
+need to set your `crate-type` accordingly:
+
+```toml
+[dependencies]
+zed_extension_api = "0.0.1"
+
+[lib]
+crate-type = ["cdylib"]
+```
+
+## Implementing an Extension
+
+To define your extension, create a type that implements the `Extension` trait, and register it.
+
+```rust
+use zed_extension_api as zed;
+
+struct MyExtension {
+    // ... state
+}
+
+impl zed::Extension for MyExtension {
+    // ...
+}
+
+zed::register_extension!(MyExtension);
+```
+
+## Testing your extension
+
+To run your extension in Zed as you're developing it:
+
+- Open the extensions view using the `zed: extensions` action in the command palette.
+- Click the `Add Dev Extension` button in the top right
+- Choose the path to your extension directory.