Move impl Interner for TyCtxt to its own submodule

[Zalathar]

This impl is several hundred lines of mostly self-contained, mostly boilerplate code that can be extracted out of the dauntingly large rustc_middle::ty::context module.

• The trait and its impl were introduced by Move things to rustc_type_ir #97287.

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There should be no change to compiler behaviour.

[rustbot]

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[Zalathar]

I had been looking into whether it's feasible to extract TyCtxt and GlobalCtxt into their own module (since context.rs is so big and incoherent), when I stumbled upon this other chunk of extractable code.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.