implement std/wrapnils without using experimental:dotOperators

[timotheecour]

avoids issues like #13063 for modules importing std/wrapnils.

when true:
  import std/wrapnils
  type Foo = object
  discard Foo().nonexistant

before PR

t11843.nim(25, 16) Error: type mismatch: got <Foo>
but expected one of:
template `.`(a: Wrapnil; b): untyped [template declared in /Users/timothee/git_clone/nim/Nim_devel/lib/std/wrapnils.nim(44, 10)]
  first type mismatch at position: 1
  required type for a: Wrapnil [compositeTypeClass declared in /Users/timothee/git_clone/nim/Nim_devel/lib/std/wrapnils.nim(44, 15)]
  but expression 'Foo()' is of type: Foo [object declared in t11843.nim(24, 8)]

after PR

t11843.nim(25, 16) Error: undeclared field: 'nonexistant' for type t11843.Foo [type declared in t11843.nim(24, 8)]
    discard Foo().nonexistant

note

experimental:dotOperators still has it's use, the fate of experimental:dotOperators is out of scope for this PR