vim-patch:9.1.0550: filetype: antlr4 files are not recognized

Problem:  filetype: antlr4 files are not recognized
Solution: Detect '*.g4' as antlr4 filetype, include a simple antlr4
          syntax and filetype plugin (Yinzuo Jiang)

closes: vim/vim#15191

4a7a4a3675

Co-authored-by: Yinzuo Jiang <jiangyinzuo@foxmail.com>

runtime/lua/vim/filetype.lua

@@ -190,6 +190,7 @@ local extension = {
   aidl = 'aidl',
   aml = 'aml',
   run = 'ampl',
+  g4 = 'antlr4',
   scpt = 'applescript',
   ino = 'arduino',
   pde = 'arduino',