Problem:
Vimscript ftplugins that inherit a base ftplugin often use explicit
`runtime! ftplugin/foo.vim` patterns. This skips corresponding Lua
ftplugins, unlike top-level ftplugin loading.

Solution:
Use the existing `{vim,lua}` patterns for bang runtime imports while
preserving each call's current lookup breadth.

Problem:  filetype: bright(er)script files are not recognized
Solution: detect *.bs files as brighterscript filetype and *.brs as
          brightscript filetype, include filetype plugins (Riley Bruins)

closes: vim/vim#17566

https://github.com/vim/vim/commit/03e5ee25fdd4f8ee16b00688e24c4ad3b8f8f935

Co-authored-by: Riley Bruins <ribru17@hotmail.com>