Rule `multiple-inline-declarations`

[LiamPattinson]

integer, parameter :: a = 1, b = 2
! Prefer:
integer, parameter :: a = 1
integer, parameter :: b = 2

This can get especially confusing when the variables on the right-hand-side have different dimensions.

However, I personally don't mind seeing this when it comes to loop variables:

integer :: i, j, k

I'm unsure if there should be a way for users to set exceptions to this rule for loop variables, file units, etc., or if that would just add too much complexity.

Should be an optional style rule, and could use a lot of utilities from #558