Type: tree-select package

[sirreal]

• Translate tree-select to TypeScript.
• Will generate typings and allow them to be published/used in Calypso.

props to @beaucollins who was the original author of most of the typings.

Testing instructions

Confirm that clean produces a clean directory (nothing in packages/tree-select/{dist,types}).
Confirm that build produces expected output (CommonJS in packages/tree-select/dist/cjs and ECMAScript Module in corresponding …/esm directory.}

Compare with most recent published builds:

• esm: https://unpkg.com/browse/@automattic/[email protected]/dist/esm/index.js
• cjs: https://unpkg.com/browse/@automattic/[email protected]/dist/cjs/index.js

The results should be similar but not identical.

# Clean
npx lerna run clean --scope="*/tree-select"
# Build
npx lerna run prepare --scope="*/tree-select"

[matticbot]

Test live: https://calypso.live/?branch=update/type-tree-select

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[sirreal]

Putting this on hold. The TS translation here is handy, but I'd like to apply my learnings when I get WordPress/gutenberg#18942 sorted.

[sirreal]

This is an unusual reducer function, it consumes an array of keys to walk a tree data structure (WeakMaps of WeakMaps) until it reaches a leaf which is a Map< string, CachedSelectorResultValue >.

I've typed map as any because usage suggests this internal function is working well enough and it was difficult to type it as Map<…> | WeakMap<…> here.

[sirreal]

I've redone this according to latest best practices in Calypso and it should be ready to move forward.

[sirreal]

Thanks for the changes and review @sarayourfriend! Since I'm a bit distanced from this work right now, I'd be most comfortable if you handle landing this at your leisure.