add custom 'toggle comment' command

ST's default 'toggle comment' command cannot handle 'block comments'
that don't have an end marker.

The new DartToggleCommentCommand class patches the default command
to also understand TM_COMMENT_BLOCK_START[_END] variables that
will forcibly be added to the 'block comment' markers, even if no
end marker is present.

This makes it possible to use /// comments by pressing Ctrl+Shift+/.

Note: This command should be removed if ST ever implements its own
way of enabling block comments/alternate comments without an end
marker.
fixes
Merge pull request #435 from my-personal-forks/fix-431-triple-slash-comments

Fix 431 triple slash comments