Investigate a mode to *remove* unused types

[JoshuaKGoldberg]

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How interesting! Idea courtesy of @sqs (superset of #34). Could TypeStat be used to remove unused types across a code base instead of adding missing ones in?

[JoshuaKGoldberg]

Some thoughts as part of the NPM milestone...

• This issue is important because --fixNoImplicitAny adds a lot of types that later on become unnecessary
• Object.keys(someObject).map((key: string | number) => {...}) seems unnecessary - it's also buggy if it infers string | number
• How to tell, for a given location, what its inferred type would be without its declaration?
• A lot of added parameter types, for example ones added to .filter/.map, end up being unnecessary
• Is there a way to get what the type would be if inferred from a variable/parameter/etc. that has a declaration?

[JoshuaKGoldberg]

A few existing mutators that will need to have this added:

• Returns
• Variable declarations
• Parameters

I'll take on this issue, since it's pretty big and might involve a lot of architecture changes.

[JoshuaKGoldberg]

Additionally, moving from someProp: any to someProp: ActualObjectType would be really useful for React conversions.

[JoshuaKGoldberg]

Thinking more on this, I see there being two fixers that could be thought of here:

• Removing inferable types, like no-inferrable-types
• Narrowing unnecessarily wide types, as discussed above