TR: add a #lang for additional "base types" that are required

This adds an internal #lang like the existing `env-lang.rkt` used for the main base type environment, except that it constructs a `#%type-decl` submodule so that the module can actually be required by a user.

The second commit shows how this is used by defining `typed/syntax/stx` which re-exports `syntax/stx` with types.