TypeScript, xo for linting, ava for testing

[osdiab]

• move preact code to subpackage, monorepo structure using yarn workspaces
• switch to typescript, type everything
  • follow up: validate input arguments into publicly exposed functions
• xo for linting
• ava for testing
  • todo: it's failing due to transpilation issues - fixed by pinning preact-cli version
• todo: build script should copy built files in subpackage docs directory to root

Current challenge: HTMLWebpackPlugin seems to not work, maybe because I upgraded preact-cli. it looks like there are two HTML webpack plugin instances so how we override appears to be broken. Not totally sure how to debug (cc @jake-nz ), related #75 , will try to revert the preact-cli bump

once that works then can do more thorough testing

closes #11
closes #17
part of #16

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[jake-nz]

Awesome! So good to get these things set up :)

[osdiab]

a lot of follow ups from this now that i've thoroughly spelunked the code, to improve its stability!

[osdiab]

tests still failing, ill figure it out in a later PR

[osdiab]

seems it thinks the docs needs updating since the preact config now results in my local directories being included in the sourcemap paths. that's odd

[osdiab]

im just gonna land this for now since it's a net benefit, and deal with the script issues after