chore: Create script to help with .js -> .ts migration

[cpcallen]

The basics

• I branched from develop
• My pull request is against develop
• My code follows the style guide
• I ran npm run format and npm run lint

The details

Proposed Changes

Add a script, js2ts, to automate some of the more trivial but annoying work to be done converting blocks and generators from JavaScript to TypeScript. In particular, it

• Converts goog.module() to goog.declareModuleId(), and adds the requisite import of goog.js.
• Converts goog.require() to import
• Converts exports.foo = foo into export foo (preferably by export to the declaration of foo if possible), rather than as a separate statement).

Reason for Changes

I'm lazy and don't want to do all this fiddly work by hand.

Test Coverage

I ran it on math/blocks.js and the result was vaguely reasonalble-looking.

Documentation

It could use some.