Add WebIDL support

WebIDL[[1]](http://www.w3.org/TR/WebIDL/) is very common in web standards.
Mozilla also use it as browser environment init configuration.
Lots of WebIDL file is already in the web world [[2]](https://github.com/mozilla/gecko-dev/tree/master/dom/webidl).
So add support for this file type.
`IDL.tmLanguage` specifies `scopeName` as `source.idl`, not `source.webidl`. This conflicts with the existing `source.idl` grammar we already have. So this grammar isn't actually usable as-is. We'll need it to use a different scope name that doesn't conflict.