Parsing and emit for computed properties

This is based on the plan in issue #1082. The type checker support, .d.ts generation and language service support (other than navigation items). I plan to work on the type system side next.
Accessors are already disallowed in ambient contexts (in the grammar).

The other stuff is not in the grammar yet because it has to do with computed properties.
Once symbols are added, the grammar will allow computed properties, but only of the form Symbol.identifier.