This makes declare class extends behave the same way as in flow
The ast-token after the extends keyword, might be either Identifier or
QualifiedTypeIdentifier
To do that this commits splits the parseGenericType into two functions,
one for parsing genericType and on for qualifiedTypeIdentifier
The primary goal of this commit is to add the ability to parse type parameter
declarations with defaults, like `type Foo<T = string> = T`. While I was in the
code, I fixed a few small things, like
* Type parameter declarations need 1 or more type parameters.
* The existential type `*` is not a valid type parameter.
* The existential type `*` is a primary type
* The param list for type parameter declarations now consists of
`TypeParameter` nodes
This has been a feature in Flow for a long time (couldn't easily find a
specific commit adding this). Interfaces are basically undocumented, though, so
it's easy to see how this was missed.
