Add label_for_completion to extension API (#10175)

This PR adds the ability for extensions to implement
`label_for_completion` to customize completions coming back from the
language server.

We've used the Gleam extension as a motivating example, adding
`label_for_completion` support to it.

Release Notes:

- N/A

---------

Co-authored-by: Max <max@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/extension_api/wit/since_v0.0.6/extension.wit

@@ -0,0 +1,120 @@
+package zed:extension;
+
+world extension {
+    import lsp;
+
+    use lsp.{completion};
+
+    export init-extension: func();
+
+    record github-release {
+        version: string,
+        assets: list<github-release-asset>,
+    }
+
+    record github-release-asset {
+        name: string,
+        download-url: string,
+    }
+
+    record github-release-options {
+        require-assets: bool,
+        pre-release: bool,
+    }
+
+    enum os {
+        mac,
+        linux,
+        windows,
+    }
+
+    enum architecture {
+        aarch64,
+        x86,
+        x8664,
+    }
+
+    enum downloaded-file-type {
+        gzip,
+        gzip-tar,
+        zip,
+        uncompressed,
+    }
+
+    variant language-server-installation-status {
+        none,
+        downloading,
+        checking-for-update,
+        failed(string),
+    }
+
+    /// Gets the current operating system and architecture
+    import current-platform: func() -> tuple<os, architecture>;
+
+    /// Get the path to the node binary used by Zed.
+    import node-binary-path: func() -> result<string, string>;
+
+    /// Gets the latest version of the given NPM package.
+    import npm-package-latest-version: func(package-name: string) -> result<string, string>;
+
+    /// Returns the installed version of the given NPM package, if it exists.
+    import npm-package-installed-version: func(package-name: string) -> result<option<string>, string>;
+
+    /// Installs the specified NPM package.
+    import npm-install-package: func(package-name: string, version: string) -> result<_, string>;
+
+    /// Gets the latest release for the given GitHub repository.
+    import latest-github-release: func(repo: string, options: github-release-options) -> result<github-release, string>;
+
+    /// Downloads a file from the given url, and saves it to the given path within the extension's
+    /// working directory. Extracts the file according to the given file type.
+    import download-file: func(url: string, file-path: string, file-type: downloaded-file-type) -> result<_, string>;
+
+    /// Makes the file at the given path executable.
+    import make-file-executable: func(filepath: string) -> result<_, string>;
+
+    /// Updates the installation status for the given language server.
+    import set-language-server-installation-status: func(language-server-name: string, status: language-server-installation-status);
+
+    type env-vars = list<tuple<string, string>>;
+
+    record command {
+        command: string,
+        args: list<string>,
+        env: env-vars,
+    }
+
+    resource worktree {
+        read-text-file: func(path: string) -> result<string, string>;
+        which: func(binary-name: string) -> option<string>;
+        shell-env: func() -> env-vars;
+    }
+
+    export language-server-command: func(language-server-id: string, worktree: borrow<worktree>) -> result<command, string>;
+    export language-server-initialization-options: func(language-server-id: string, worktree: borrow<worktree>) -> result<option<string>, string>;
+
+    record code-label {
+        /// The source code to parse with Tree-sitter.
+        code: string,
+        spans: list<code-label-span>,
+        filter-range: range,
+    }
+
+    variant code-label-span {
+        /// A range into the parsed code.
+        code-range(range),
+        literal(code-label-span-literal),
+    }
+
+    record code-label-span-literal {
+        text: string,
+        highlight-name: option<string>,
+    }
+
+    record range {
+        start: u32,
+        end: u32,
+    }
+
+    export labels-for-completions: func(language-server-id: string, completions: list<completion>) -> result<list<option<code-label>>, string>;
+}

crates/extension_api/wit/since_v0.0.6/lsp.wit

@@ -0,0 +1,44 @@
+interface lsp {
+    /// An LSP completion.
+    record completion {
+        label: string,
+        detail: option<string>,
+        kind: option<completion-kind>,
+        insert-text-format: option<insert-text-format>,
+    }
+
+    variant completion-kind {
+        text,
+        method,
+        function,
+        %constructor,
+        field,
+        variable,
+        class,
+        %interface,
+        module,
+        property,
+        unit,
+        value,
+        %enum,
+        keyword,
+        snippet,
+        color,
+        file,
+        reference,
+        folder,
+        enum-member,
+        constant,
+        struct,
+        event,
+        operator,
+        type-parameter,
+        other(s32),
+    }
+
+    variant insert-text-format {
+        plain-text,
+        snippet,
+        other(s32),
+    }
+}