Add ansi256 and bgAnsi256 to TypeScript declarations

[pedrottimark]

When I test bgAnsi256 as visual enhancement, 2.4.2 and 3.0.0-beta.1 have TypeScript errors:

ansi256.ts:3:30 - error TS2339: Property 'bgAnsi256' does not exist on type 'Chalk'.

3 const aColor = chalk.magenta.bgAnsi256(225);
                               ~~~~~~~~~

ansi256.ts:4:28 - error TS2339: Property 'bgAnsi256' does not exist on type 'Chalk'.

4 const bColor = chalk.green.bgAnsi256(194);

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
4. Delete extra space in neighbor line of readme.md

[sindresorhus]

https://github.com/chalk/chalk/releases/tag/v3.0.0-beta.2