mpv

### What is the Addon for?

Add definitions for the [mpv lua module](https://mpv.io/manual/master/#lua-scripting).

### Additional Notes

MPV (commandline video player) provides lua scripting capabilities and an integrated lua module.

Start with the linked mpv Lua scripting manual and identify the module’s exposed API. Then inspect existing LLS-Addons definitions to follow the project’s addon structure and add matching mpv definitions; done means the documented module API is available to Lua language server users.

Written by the indexing model from the issue text.