[SyntaxTree] Use simplified grammar rule for `NestedNameSpecifier` grammar nodes

This is our grammar rule for nested-name-specifiers:
globalbal-specifier:
  /*empty*/
simple-template-specifier:
  template_opt simple-template-id
name-specifier:
  global-specifier
  decltype-specifier
  identifier
  simple-template-specifier
nested-name-specifier:
  list(name-specifier, ::, non-empty, terminated)

It is a relaxed version of C++ [expr.prim.id] and quite simpler to map to our API.

TODO: refine name specifiers, `simple-template-name-specifier` and
decltype-name-specifier` are token soup for now.