vim-patch:9.1.0425: filetype: purescript files are not recognized

Problem:  filetype: purescript files are not recognized
Solution: recognize '*.purs' files as purescript filetype,
          include basic purescript filetype plugin
          (Riley Bruins)

Reference: https://github.com/purescript/documentation/blob/master/language/Syntax.md#comments

closes: vim/vim#14813

155583a5c3

Co-authored-by: Riley Bruins <ribru17@hotmail.com>

runtime/lua/vim/filetype.lua

@@ -845,6 +845,7 @@ local extension = {
   psf = 'psf',
   psl = 'psl',
   pug = 'pug',
+  purs = 'purescript',
   arr = 'pyret',
   pxd = 'pyrex',
   pyx = 'pyrex',