fix(ftplugin): source inherited Lua ftplugins

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.

runtime/ftplugin/brighterscript.vim

@@ -8,4 +8,4 @@ if exists('b:did_ftplugin')
 endif
 let b:did_ftplugin = 1
 
-runtime! ftplugin/brightscript.vim
+runtime! ftplugin/brightscript[.]{vim,lua}