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/src/extension_api.rs

@@ -1,6 +1,7 @@
 //! The Zed Rust Extension API allows you write extensions for [Zed](https://zed.dev/) in Rust.
 
 pub mod http_client;
+pub mod process;
 pub mod settings;
 
 use core::fmt;