Problem: filetype: antlr4 files are not recognized
Solution: Detect '*.g4' as antlr4 filetype, include a simple antlr4
syntax and filetype plugin (Yinzuo Jiang)
closes: #15191
Signed-off-by: Yinzuo Jiang <jiangyinzuo@foxmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
