Remove the 'Method' syntaxkind and introduce MethodSignature and MethodDeclaration instead.

Sharing a single kind here would be bad for incremental scenarios as these two node types
are not interchangeable.

For now, i have updated nearly all code to look for both kinds.  However, it may not make sense
in all locations, and we could likely tighten up many code locations to only have to deal with
one or the other.

--

Note: i need to do this for PropertyDeclaration/PropertySignature as well.  Will be pushing that change shortly.