Clean up code. Reorganize files. Port stuff from other branches. + more

This turns the project into a monorepo using pnpm workspaces,
dramatically simplifying the build process. It also fixes a lot of bugs
and just generally makes the codebase a lot cleaner.

pnpm-workspace.yaml

@@ -0,0 +1,10 @@
+packages:
+  - packages/*
+  - example-app/
+
+onlyBuiltDependencies:
+  - '@parcel/watcher'
+  - core-js-pure
+  - esbuild
+
+shamefully-hoist: true