patch 9.0.2128: runtime(swig): add syntax and filetype plugins

patch 9.0.2128: runtime(swig): add syntax and filetype plugins

Add syntax and filetype plugins for SWIG (Simplified Wrapper Interface
Generator) description files.

The default syntax for .i files highlights comments in a reverse
color scheme which doesn't look well.  This syntax builds
on vim's c++ syntax by adding highlighting for common swig
directives and user defined directives.  For an alternative
syntax, see vimscript #1247 (which I found after writing this).

closes: #13562

Co-authored-by: Matěj Cepl <mcepl@cepl.eu>
Co-authored-by: Julien Marrec <julien.marrec@gmail.com>
Signed-off-by: Julien Marrec <julien.marrec@gmail.com>
Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>