[dev.typeparams] go/parser: accept embedded type literals

This is an approximate port of CL 321109 to go/parser, though go/parser
does not have the same internal APIs as cmd/compile/internal/syntax, so
this CL required some refactoring.

Change-Id: I146ef530c969d61bab99f98f4de94b862e103ddc
Reviewed-on: https://go-review.googlesource.com/c/go/+/325703


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>