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 vim/vim#1247 (which I found after writing this).
closes: vim/vim#135622e31065a65
Co-authored-by: Julien Marrec <julien.marrec@gmail.com>
Co-authored-by: Matěj Cepl <mcepl@cepl.eu>
