make REPL ast transforms instance-specific

[stevengj]

As suggested here, this makes the REPL ast transformation list specific to a REPLBackend instance, though there is a global array of defaults for new instances.

This PR also changes REPL.softscope! to REPL.sofscope. There doesn't seem to be a big advantage to having this function mutating, and the non-mutating version is easier to use safely. (In particular, though passing the mutating version to the new include function in #34595 is safe because the AST expressions are temporary, that would require us to document this property of include which is bothersome.)

[fredrikekre]

This PR also changes REPL.softscope! to REPL.sofscope.

Documenter has been patched to use softscope! so in order to get the doc builder to pass on this PR you will have to revert 2c43528

[StefanKarpinski]

Maybe the least disruptive course of action would be:

1. make softscope! and alias for softscope for the time being
   • I doubt anyone was relying on the side effect, they were probably mapping it
2. change Documenter to call softscope instead of softscope!
3. bump the Documenter version that Base uses
4. delete the softscope! alias

[stevengj]

What is the remaining doctest error? I'm not sure what is going on…

[KristofferC]

It has been fixed on master but this needs to be rebased to pick it up (CI don't run merge commits now).

[stevengj]

Hooray, green tests!