Act as a thumbfast provider

[po5]

When the patched OSC is renamed to thumbfast.lua, it will expose a thumbfast-compatible API. The script returns and doesn't act as an OSC.
This allows the thumbnailer to work with any UIs that implement this less invasive API: https://github.com/po5/thumbfast#ui-support
Positioning and decorations are left up to each client script.
Renaming is required as all scripts use script-message-to thumbfast, which mpv forwards only to the specified script.
Of course it still works in its normal non-thumbfast mode.

Closes #1

Hoping this will spark renewed interest and we can implement each other's fixes to improve thumbnails.
I am the maintainer of thumbfast, if that wasn't obvious.

[Alistair1231]

I rebased this onto the newest upstream changes and created a release for it if anyone is curious:
https://github.com/Alistair1231/mpv_thumbnail_script/releases/