fix(client): Enum type clashes

[SevInf]

Follow up to #20150.
Changes we did to extensions have fixed name clashes for models, but not
for enums. Enums problem is more complicated: in case of name clash, it
is impossible for contents of Prisma namespace to reference the value
from outer scope.
Solution is: all top-level enums are put into $Enums namespace. Types
within Prisma namespace always refer to them using fully-qualifed
name. Top-level enums are re-exported from $Enums namespace for
backward compatiblity.

Also, does opportunistic clean-ups/refactors

Fix #20031

[codspeed-hq]

CodSpeed Performance Report

Merging #20194 will degrade performances by 51.82%

_{Comparing fix/enum-type-clash (b4bf636) with main (99a96e8)}

Summary

❌ 1 regressions
✅ 3 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	fix/enum-type-clash	Change
❌	typescript compilation ~50 Models	77.4 ms	160.6 ms	-51.82%