zed_extension_api: Add simple process API (#25399)

This PR adds a simple API for working with processes to the extension
API.

The API is designed to mirror Rust's
[`std::process::Command`](https://doc.rust-lang.org/std/process/struct.Command.html).

Release Notes:

- N/A

crates/extension_api/wit/since_v0.3.0/common.wit

@@ -6,4 +6,7 @@ interface common {
         /// The end of the range (exclusive).
         end: u32,
     }
+
+    /// A list of environment variables.
+    type env-vars = list<tuple<string, string>>;
 }

crates/extension_api/wit/since_v0.3.0/extension.wit

@@ -4,10 +4,12 @@ world extension {
     import github;
     import http-client;
     import platform;
+    import process;
     import nodejs;
 
-    use common.{range};
+    use common.{env-vars, range};
     use lsp.{completion, symbol};
+    use process.{command};
     use slash-command.{slash-command, slash-command-argument-completion, slash-command-output};
 
     /// Initializes the extension.
@@ -56,19 +58,6 @@ world extension {
     /// 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);
 
-    /// A list of environment variables.
-    type env-vars = list<tuple<string, string>>;
-
-    /// A command.
-    record command {
-        /// The command to execute.
-        command: string,
-        /// The arguments to pass to the command.
-        args: list<string>,
-        /// The environment variables to set for the command.
-        env: env-vars,
-    }
-
     /// A Zed worktree.
     resource worktree {
         /// Returns the ID of the worktree.

crates/extension_api/wit/since_v0.3.0/process.wit

@@ -0,0 +1,29 @@
+interface process {
+    use common.{env-vars};
+
+    /// A command.
+    record command {
+        /// The command to execute.
+        command: string,
+        /// The arguments to pass to the command.
+        args: list<string>,
+        /// The environment variables to set for the command.
+        env: env-vars,
+    }
+
+    /// The output of a finished process.
+    record output {
+        /// The status (exit code) of the process.
+        ///
+        /// On Unix, this will be `None` if the process was terminated by a signal.
+        status: option<s32>,
+        /// The data that the process wrote to stdout.
+        stdout: list<u8>,
+        /// The data that the process wrote to stderr.
+        stderr: list<u8>,
+    }
+
+    /// Executes the given command as a child process, waiting for it to finish
+    /// and collecting all of its output.
+    run-command: func(command: command) -> result<output, string>;
+}