tsdoc comments and readme for @polywrap/client-js

[krisbitney]

This PR adds comments in the TSDoc format to public methods of the JS PolywrapClient, and adds a readme with full reference documentation based on the comments.

The benefits of this approach:

1. The "Reference" section of the readme is easy to create after copy/pasting the .d.ts file, which contains the comments. Although I did this part manually, it could be automated with a script.
2. We can promote a process wherein developers are expected to update or add tsdoc comments when modifying javascript libraries.

This makes it easy for us to keep reference documentation up to date.

Related: Documentation Roadmap (hackmd) Summary of Documentation Roadmap (discord)

Closes https://github.com/polywrap/documentation/issues/206