Implement toolkit API

[pcardune]

This PR attempts to refactor and consolidate all the code and configuration that was copy/pasted across codemirror-blocks, wescheme-blocks, and pyret-blocks into a small "toolkit" library that lives in "src/toolkit". Here is a summary of the changes:

• npm run build will generate a lib/ directory containing transpiled commonjs versions of all the files in src/, allowing dependents to import individual files from codemirror-blocks with require("codemirror-blocks/lib/<filename>");
• we will no longer generate a dist/ directory with CodeMirrorBlocks-min.js bundles. This responsibility was long ago pushed to pyret-blocks and wescheme-blocks
• common code and configuration for running/writing tests, generating bundles, and running a local development server will live in "src/toolkit" and be accessible from wescheme-blocks and pyret-blocks via require("codemirror-blocks/lib/toolkit/<module>")

TODO BEFORE MERGE:

• delete webpack bundling configs from webpack/ directory since we will stop bundling this repo
• move spec/support to src/toolkit
• Update wescheme-blocks to use this api (See Cleanup copy/pasta and use new codemirror-blocks toolkit API wescheme-blocks#23)
• Update pyret-blocks to use this api (See Get pyret-blocks working with toolkit branch of codemirror-blocks pyret-blocks#30)
• Review doc comments for all functions exposed from src/toolkit

[pcardune]

@schanzer I believe this is ready to merge. There are more changes to be made in pyret-blocks and wescheme-blocks, but my pull requests in those repos address the main compatibility issues that would prevent this PR from being merged.

[schanzer]

@pcardune this is huge. Thank you!!

Before I merge, I want to confirm that you're intentionally leaving one of the TODO items unchecked? I'm also curious about why the extendSelectionX apis are commented out in BlockEditor.tsx, as it looks like you did some work on their implementation.

[pcardune]

Yes, I'm intentionally leaving one of the TODO items unchecked.

The extendSelectionX apis are commented out in BlockEditor.tsx because they are in fact supported by the block editor. I commented them out from the list of "unsupported apis" instead of just removing them from the list to match the on and off APIs.

I'll go ahead and merge this PR and the corresponding PRs for wescheme-blocks and pyret-blocks. Since the pyret-blocks and wescheme-blocks repos have always pointed to the latest commit in the codemirror-blocks master branch instead of a specific commit hash, all old commits to wescheme-blocks and pyret-blocks will probably be broken because they will be pointing to a version of codemirror-blocks that is no longer compatible. Likewise, any outstanding branches in wescheme-blocks and pyret-blocks will probably also break until they merge master.