Implement API to cancel async jobs

Co-authored-by: sfan5 <sfan5@live.de>

doc/lua_api.md

@@ -7156,12 +7156,12 @@ This allows you easy interoperability for delegating work to jobs.
     * When `func` returns the callback is called (in the normal environment)
       with all of the return values as arguments.
     * Optional: Variable number of arguments that are passed to `func`
+    * Returns an `AsyncJob` async job.
 * `core.register_async_dofile(path)`:
     * Register a path to a Lua file to be imported when an async environment
       is initialized. You can use this to preload code which you can then call
       later using `core.handle_async()`.
 
-
 ### List of APIs available in an async environment
 
 Classes:
@@ -7992,6 +7992,14 @@ use the provided load and write functions for this.
 * `from_file(filename)`: Experimental. Like `from_string()`, but reads the data
   from a file.
 
+`AsyncJob`
+----------
+An `AsyncJob` is a reference to a job to be run in an async environment.
+
+### Methods
+* `cancel()`: try to cancel the job
+    * Returns whether the job was cancelled.
+
 `InvRef`
 --------