This helps those who want to contribute to the TypeScript definitions
by adding:
* a tool called `tsd,`
* its corresponding configuration in package.json
* a teste file index.test-d.ts
Also add a extra line between type definitions to improve readability.
* chain.d.ts: add options argument to put & set
* chain.d.ts: set cb may be null when providing opts
Co-authored-by: Norman Reed <norman.s.reed@gmail.com>