Add docs generator command for extension authors

[riknoll]

i would love to have a command that takes JSDoc from extension APIs and generates markdown files for all exported blocks in the format described here: https://makecode.com/extensions/github-docs

it should:

1. find all APIs with //% block (can skip deprecated blocks)
2. have an option to automatically add the //% help="" annotation since it always confuses everyone
3. add empty JSDoc with all the params to the API if it doesn't already exist
4. create a markdown file inside a docs/ folder if it doesn't exist
5. populate that markdown file with the contents of the JSDoc (if present)

all of these options/steps should be controllable with flags

[pelikhan]

also generate the jsdoc/block string json files using mkc