Watch mode: automatically rebuild when there are changes to the file system

Watch *source-path*, *library-path*, *external-library-path* and *asconfig.json* for changes and rebuild.

Probably using this package: https://www.npmjs.com/package/chokidar

EDIT: Royale's compiler natively supports a `--watch` compiler option. It is supported with asconfigc's `--watch` option since asconfigc 1.6.0. See commit a3209fd88793e66e10059de128431e096fb80e34. Other compilers will require a custom solution.

Start with asconfigc’s --watch option and commit a3209fd88793e66e10059de128431e096fb80e34, then review how source-path, library-path, external-library-path, and asconfig.json are handled. Done means supported compilers rebuild after relevant file-system changes, while other compilers have an identified custom-watch behavior.

Written by the indexing model from the issue text.