Relax default overload subtyping checks

Currently, mypy ensures monotnonically increasing order when testing overloads against overloads:

https://github.com/python/mypy/blob/9f157d988f4da3624b0c58a2e7dd5f777bc8a755/mypy/subtypes.py#L1003-L1009

However, I believe (1) this rule does more harm than good in many cases when it is correct, and (2) there are many cases where it is incorrect. Therefore, I propose to move this check behind a new strictness flag and disable it by default. Later, one can see if the check can be refined to deal with the cases listed below.

Rationale

In https://github.com/python/typing/issues/2021 I showed that the current typing spec is too strict with respect to subtyping overloads.

When this check is incorrect

The check is incorrect when overloads are commutative, i.e. changing the overload order describes the same callable type. This is the case when:

1. Argument signatures are mutually exclusive mypy-playground

2. Argument types are mutually exclusive (e.g. via @disjoint_bases) mypy-playground

3. Argument types overlap, but return types are identical. mypy-playground

When this check is technically correct, but hurts in practice

As @disjoint_bases is relatively new, many libraries likely do not use it. Moreover, Intersection types are still not supported. Thus, many classes that are compatible at runtime cannot be put under the umbrella of a shared Protocol because inadvertently they will use different overload order at one point or another.

Start in mypy/subtypes.py at lines 1003-1009 and inspect how overloads are checked against overloads. Review the linked typing discussion and examples, then determine the strictness flag behavior and verify that the check is disabled by default while remaining available under the flag.