Add an -Yimplicit-to-given flag for rewrites to easily test changes in the ecosystem

[jchyb]

closes #22482
It replaces every implicit definition with given (including implicit classes, which I was surprised to see working), and every implicit parameter clause with using, all in the parser phase.
Symbols and trees unpickled from tasty are unaffected by this flag. Trees generated using the reflection api are unaffected as well.
Pre -source:3.6 context bounds are left as implicits.
All of the above should be easy enough to change, but I didn't really see the purpose for it.

As I run into several issues with the previous method (where we would try to interpret implicit keywords as givens, with no changes to source code, which could lead to unclear error messages, and different behavior in dependent modules), I ended up redoing this to be rewrite-based. Now, the added flag should be used in conjunction with --rewrite, adding rewrite patches to the source code. This will omit implicit classes and old-style implicit conversions, where a simple rewrite to given or using might not be available. using keywords are also added to converted previously-implicit-now-using parameter applications, but only if both the method definition and application are in the same compilation run (in other cases they would have to be added manually).

[odersky]

I think it would be a good idea to merge this. It can be helpful for migrating to givens.

[jchyb]

We probably should not merge this yet, as last time @WojciechMazur tested this with the cats repository, he detected some more differences beyond the given/implicit resolution unnaccaunted for here (e.g. implicits allow an additional empty argument list, but when replacing that with using, we get an error unrelated to what we want to be tested here).

I'll try to fix those issues and will prioritize this PR over the coming days, so that this can be merged (also since we want this merged in the compiler, I'll probably need to replace the added symbol flag with something else)