Add ansi and bgAnsi to TypeScript declaration

[pedrottimark]

Here are remaining methods, after I researched the meaning of the argument:

1. Add 2 type tests to index.test-d.ts which failed first
2. Add 2 declarations to index.d.ts
3. Add link and example to readme.md

Thank you very much for quick turn-around on #368 ❤️

[pedrottimark]

What do y’all think about removing these 2 methods from chalk public interface?

Through research and discussion in #370 I see internal use of ansi to convert down to lower-level format, but it seems like SGR color code as argument is a limited use case for chalk callers:

• first I jumped to assumption that argument is 4-bit unsigned color index, like 1 for red
• then I jumped to assumption that at level 1 the argument could be any SGR code, like 1 for bold, but the close code does not match: default color not normal weight, or whatever else

[sindresorhus]

@pedrottimark Can you move the above into a new issue? The Chalk v3 release is imminent, but we can discuss whether to remove them for the next major version.