[wgsl-in] Handle modf and frexp

[fornwall]

• Fixes Change frexp and modf to work with structures #1161
• Fixes Figure out modf story #594 (this is outdated/a duplicate, and can be closed regardless of this PR)
• Replaces Change frexp and modf to return structures #1441

Another approach than what is done here would be to use TypeInner::Struct to reuse struct handling logic. There are some changes necessary there to handle naming correctly, and e.g. avoid writing out the struct in wgsl-out (which already has the struct defined).

Let med know if you want that done instead (and if so, how to proceed there - perhaps adding something like a enum StructType { UserDefined, ModfResult, FrexpResult } (and perhaps AtomicCompareExchangeWeaResult?) to TypeInner::Struct)!

[fornwall]

Replaced by a TypeInner::Struct using variant (#2453).