Follow-up to #8893 (comment)

We need to decide:

• Should we freeze the webrepl Python files on all boards that include the webrepl C support (which is now controlled by MICROPY_PY_NETWORK)
• or. Make webrepl something that can be installed optionally (upip, etc) and document that, and leave it to specific boards that want to freeze it.

(Following our general principle of only including the bare minimum by default, I have a weak preference for the second option, especially if we make package installation more seamless... see notes in #8914)