Add `move_const_to_impl` assist
Closes#13277
For the initial implementation, this assist:
- only applies to inherent impl. Much as we can *technically* provide this assist for default impl in trait definitions, it'd be complicated to get it right.
- may break code when the const's name collides with an item of a trait the self type implements.
Comments in the code explain those caveats in a bit more detail.
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