First draft for in_linter()

[Bisaloo]

First attempt to fix #1875.

Questions:

• Should scalar_in_linter() be merged with this linter? I see these two linters are counterpart to one another.

Unaddressed cases:

• Mixed chains such as x == 'a' | y == 'b' | x == 'c' are not covered at this time but I would assume most people at least group the tests by variable(?) so it's maybe okay to leave it for now.

[AshesITR]

I think it might be nice to be able to tweak a minimum nesting depth, i.e., number of repeated | x == "val" expressions, especially since experimentally, the two-component case is slower with %in%.

Also note there is different behavior with respect to NA inputs.