feat: untangle quartz from local configs in least amount of changes

[necauqua]

For the current setup where people have to fork or at least clone quartz this changes nothing - but it allows you to install quartz as a devDependency via npm and have it actually work.

One real change is switch from .quartz-cache to
node_modules/.cache/quartz for transpilation results, this is an artifact from my previous attempts, I guess with this one I can change it back - but node_modules/.cache feels more better imo idk.

edit: OTOH if you want to have quartz be a completely separate binary (which this also enables I think), having it create a node_modules folder is weird, so I made a quick hack for that for now.

Example:

$ mkdir my-repo && cd my-repo
$ npm i quartz@necauqua/quartz#untangled # quartz@ prefix is important
$ cp node_modules/quartz/quartz.*.ts .   # copy the default configs
$ mkdir content && echo "# Hello World!" > content/index.md
$ npx quartz build --serve # this just works!
$ echo 'body { background: red !important; }' > styles.scss

Notice how I used my branch in the npm i line, ideally it'd be npm i quartz@jackyzha0/quartz, or maybe we can somehow get the quartz package on npm and it'll just be npm i quartz.
In the latter case npx quartz build will literally just work without a local npm package at all?.

Having some support for components and plugins being in separate npm packages instead of people copying code around is not out of the picture with this too btw.

Closes #502

[necauqua]

Ugh and windows strikes the dream down (cuz not sure what it doesn't like and don't have a win machine to test - time to spin up a vm?.) ah wait it's just path separator bs, one moment

Also looking at diffs I realised we can just use the import cfg from "$config" spell everywhere and remove cfg from BuildCtx, which will simplify workers, cuz cfg was the only reason we had to reconstruct BuildCtx in an annoying way.