@turf/invariant getTypes cleanup

[mfedderly]

Ran into some issues when typing @turf/line-split that warranted a small cleanup PR.

• Make type inference work a lot better
• Simplify logic
• Add a test for Feature behavior, which is surprising and I don't like it
• Remove an unused parameter

I'm kind of unsure how much of a breaking change this is. Removing a parameter that was never used breaks the signature, but also I'm not sure how anyone would've been passing in anything for this parameter that appears to have been unused since we started shipping types.

We're also narrowing the return type from string to a specific string (or string union, depending on input). This could cause a new break with something like the (very contrived) snippet below:

let t = getType(pointFeature); // t is of type `"Point"` now, previously it would have been `string`
t = "foo"; // previously that's fine, but now it would error with "foo" isn't "Point"

Overall I think I'd push for merging this in 7.x, but happy for pushback there if others feel strongly against.