Add missing `If` types for `Is` types

[ikelax]

This PR introduces the missing If types for the following Is types:

• IsNegative
• IsInteger
• IsFloat
• IsEqual
• The Is types for literal types in source/is-literal.d.ts

I have followed the contributing guidelines. I also plan to review one of the open pull requests, as recommended.

Question:

I noticed typos in source/is-integer.d.ts on lines 6 and 11 (is a integer and is a intrger). Would you prefer a separate PR to address these, or should I include the fix in this one?

Closes #788.