integrate l3docstrip module support in docstrip

[FrankMittelbach]

Brief outline of the enhancement

l3docstrip and docstrip are nearly identical. The only difference is that l3docstrip can deal with the "module" convention, i.e.

%<@@=module>

While this is only useful in expl3 code it is nevertheless pointless have two extraction programs when one can do. So the proposal is to integrate that support into docstrip.tex and promote to use that always (with l3docstrip kept available as a trial shell calling dostrip).

[JohannesBraams]

There are two parts to this:
1 first integrate the <@@=...> functionality into docstrip.dtx; test and verify it
2 strip l3docstrip to just load the new docstrip.tex

[FrankMittelbach]

• 2a test if format is 2020/10/01 or later
• 2b if not, keep what is currently added in l3docstrip