Permit variable declaration under "or patterns".

[Happypig375]

According to https://github.com/dotnet/csharplang/blob/6901635c383801e4d177085587aaccadaa7b2f11/proposals/patterns3.md#variable-definitions-and-definite-assignment, we will not be able to declare pattern variables under "or patterns".
Obviously this should be supported. We should at least allow cases where all branches of the "or pattern" declare the same number of variables of the same types. (For variables of different types, we should wait for #399.)

An example that this can simplify is

https://github.com/verybadcat/CSharpMath/blob/b8a5ab0ff9789627edf2f05927291a87631b29a0/CSharpMath.Evaluation/Evaluation.cs#L395-L403

The when clause can be simplified by #3435, but we still need to permit variable declaration under "or patterns" to merge the two cases into one and eliminate the unnecessary goto.

[jnm2]

I'd like the ability to declare variables on either side of || too:

return value is TInner inner || innerConverter.ConvertFrom(context, culture, value) is TInner inner
    ? Wrap(inner)
    : base.ConvertFrom(context, culture, value);

[huoyaoyuan]

I'd like the ability to declare variables on either side of || too:

return value is TInner inner || innerConverter.ConvertFrom(context, culture, value) is TInner inner
    ? Wrap(inner)
    : base.ConvertFrom(context, culture, value);

((value as TInner) ?? (innerConverter.ConvertFrom(context, culture, value) as TInner)) is {} inner

Well you do have an alternative, and I'd say they are both ugly syntax.

[jnm2]

I can't use as because TInner is not constrained to class.

[YairHalberstadt]

Closing as championed at #4018