Proposal: More flexible SELECT TYPE

Something so we can avoid these annoying nested `select type` blocks.

So if you want to check if several variables are integers:

```fortran
! if several variables are integers
select type (a,b,c) ! new syntax
class is (integer)
d = a + b + c
end select
```

Or if you want to check if a variable is any integer kind:

```fortran
select type (a)
class is (integer(kind=*)) ! new syntax
d = a * 2
end select
```

Or any numeric kind:

```fortran
select type (a)
class is (integer(kind=*), real(kind=*)) ! new syntax
d = 1 * 2
end select
```

And all together:

```fortran
select type (a,b,c) ! new syntax
class is (integer(kind=*), real(kind=*)) ! new syntax
! a, b, and c are all either integers or reals,
! so compiler knows what to do here:
d = a + b + c
end select
```

Start by reviewing the proposed SELECT TYPE examples in the issue and the relevant Fortran standard rules. No repository files or tests are identified, so first determine where a language proposal should be specified and validated. Done means reaching agreement on the syntax and semantics for multiple variables, wildcard kinds, and multiple intrinsic types.

Written by the indexing model from the issue text.