[2025-06 LWG Motion 34] P3552R3 Add a Coroutine Task

[dietmarkuehl]

I have applied the changes for P3552r3 (Add a Coroutine Task Type). Compared to the actual text of P3552r3
I made some [I think editorial] changes:

• added with_error and change_coroutine_scheduler declarations to the synopsis before task; they are in the detailed description but not in the synopsis (possibly they should only be in the synopsis) as there is no further description)
• inconsistent default template arguments for the ctor in the class declaration (allocator and the later description (allocator); the wording consistently uses allocator
• added "of the" to a sentence about the type of a completion datum: "... defines the type of the value completion datum"
• the use of the template parameter of the nested type state is Rcvr which has two problems: 1. the ctor also uses the name Rcvr for its template parameter 2. the class stores an object of type R which is the receiver named rcvr the most logic fix is to use Rcvr consistently for the state and change the name for the ctor's parameter:
  1. change the ctor template parameter type to use R
  2. change the member to use Rcvr
  3. change the type R used for the own-env-t to be "...get_env(declval())..."

Fixes #7971
Fixes cplusplus/papers#2200