[cpp.predefined] Feature-test macro for `consteval` blocks

[frederick-vs-ja]

Full name of submitter (unless configured in github; will be published with the issue): Jiang An

Reference (section label): [cpp.predefined]

Link to reflector thread (if any):

Issue description:

P3289R1 proposed to add feature-test macro __cpp_consteval_block. But when it was merged into P2996, the FTM was dropped. It is not clear whether we want a particular FTM for consteval blocks.

Suggested resolution:

Add a row to [tab:cpp.predefined.ft]:

Macro name	Value
[...]	[...]
__cpp_consteval_block	202506L
[...]	[...]