Modernise

[Rich-Harris]

• switch to pnpm
• switch to ESM
• get rid of the build step (except tsc)
• get rid of mocha in favour of node:test
• add prettier
• add github CI
• add pkg.exports

there's way more that can be done (i haven't changed any of the API, even though it's arguably pretty wacky by modern standards) but i don't want to sink too much time into this

[Rich-Harris]

got some windows failures but i'm not going to look into them now, i'm done for the day

[benmccann]

Just FYI, Simon managed to rip sorcery out of svelte-preprocess, so you don't necessarily need to bang your head on this too long if it ends up being troublesome

[Rich-Harris]

I know, but I've come this far...