vim-patch:9.1.1936: filetype: Erlang lexical files are not recognized

Problem:  filetype: Erlang lexical files are not recognized
Solution: Detect *.xrl files as leex filetype, include syntax and
          filetype plugins (Jon Parise).

leex is the lexical analyzer generator for Erlang. Its input file format
follows a section-based structure and uses the `.xrl` file extension.

This initial work includes file detection, an ftplugin (which inherits
the Erlang configuration), and a syntax definition.

Reference:
-  https://www.erlang.org/doc/apps/parsetools/leex.html

related: vim/vim#18819
closes: vim/vim#18832

b087c5452b

Co-authored-by: Jon Parise <jon@indelible.org>

runtime/lua/vim/filetype.lua

@@ -705,6 +705,7 @@ local extension = {
   journal = 'ledger',
   ldg = 'ledger',
   ledger = 'ledger',
+  xrl = 'leex',
   leo = 'leo',
   less = 'less',
   lex = 'lex',