effects: add new `@consistent_overlay` macro (#54322)
This PR serves to replace #51080 and close #52940.
It extends the `:nonoverlayed` to `UInt8` and introduces the
`CONSISTENT_OVERLAY` effect bit, allowing for concrete evaluation of
overlay methods using the original non-overlayed counterparts when
applied. This newly added `:nonoverlayed`-bit is enabled through the
newly added `Base.Experimental.@consistent_overlay mt def` macro.
`@consistent_overlay` is similar to `@overlay`, but it sets the
`:nonoverlayed`-bit to `CONSISTENT_OVERLAY` for the target method
definition, allowing the method to be concrete-evaluated.
To use this feature safely, I have also added quite precise
documentation to `@consistent_overlay`.
