Add a 'Word' (64 bits) primitive type

I was wondering if we could have a new `Word` primitive type that would:
1. use `0wDEADBEEFCAFEBABE` literals (saw that SML had those, I'm willing to bikeshed the syntax),
2. be 64-bit unsigned on _all_ platforms (this would likely include some JS shim, or "polyfill" as they call it -- either wrapping a pair of `number`s or using `bigint`),
3. supported only casts into/from existing numeric types, and
4. all bitwise operations would need to be done on `Word`s or `Byte`s, that is no more bitshifts on `Int`s.

This would be useful for both FFI _and_ for hashing, more bit-twiddling, bitset-ey structures, etc.

_Originally posted in https://github.com/effekt-lang/effekt/issues/1403#issuecomment-5208819571_

No files, tests, or entry points are named. Start by mapping where primitive types, literal syntax, numeric casts, and bitwise operations are defined, then resolve the literal and cross-platform representation choices before implementation; done means the proposed Word behavior is specified across all listed operations and platforms.

Written by the indexing model from the issue text.