vim-patch:9.0.2060: *.{gn,gni} files are not recognized

Problem:  *.{gn,gni} files are not recognized
Solution: Detect some as gn filetype (without adding an extra filetype)

Those come from: https://gn.googlesource.com/gn/

closes: vim/vim#13405

84394f2be4

Co-authored-by: Amaan Qureshi <amaanq12@gmail.com>

runtime/lua/vim/filetype.lua

@@ -459,6 +459,8 @@ local extension = {
   gift = 'gift',
   gleam = 'gleam',
   glsl = 'glsl',
+  gn = 'gn',
+  gni = 'gn',
   gpi = 'gnuplot',
   go = 'go',
   gp = 'gp',