[ty] support enum `_value_` annotation (#22228)
## Summary
The` _value_` attribute is used inside an Enum class to explicitly
define the underlying value of an enum member. Typing can be verified on
the members.
## Test Plan
Added a mdtest
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Co-authored-by: Carl Meyer <carl@astral.sh>
Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>
