patch 9.1.1936: filetype: Erlang lexical files are not recognized

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: #18819
closes: #18832

Signed-off-by: Jon Parise <jon@indelible.org>
Signed-off-by: Csaba Hoch <csaba.hoch@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>