Populate matches.syn, add SyncludeIncludeMatch().

Add:
	vim/scripts/Synclude/plugin/Synclude.vim
		-Move logic from `Synclude` command to `SyncludeIncludeMatch()`,
		which additionally verifies whether a given match is defined,
		and `echom` a warning otherwise.

	matches.syn, vim/ftplugin/(c, python).vim
		-Populate `matches.syn` with syntax matches shared by `c.vim`
		and `python.vim`; replace their definitions in either of the
		latter two files with `Synclude`.