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/freebasic.vim

@@ -11,7 +11,7 @@ endif
 let s:cpo_save = &cpo
 set cpo&vim
 
-runtime! ftplugin/basic.vim
+runtime! ftplugin/basic[.]{vim,lua}
 
 let s:dialect = freebasic#GetDialect()