switch to pnpm

[William Stein]

This is #50

The main significant remaining issues is that we need to automate building the dependencies, since
sibling packages are now installed directly, rather than from npm. This happens automatically and
is awesome! It does mean some additional work. Maybe a script that just automates building deps,
as determined by parsing what is installed.

Also, we'll need to update the docs, and make sure pnpm is installed. We might instead use pnpm env to
have pnpm install node instead. Hmmm...

This problem of building the deps in a workspace also comes up in CoCalc and isn't totally solved there either.
For that it's maybe a lot harder due to circular deps? In any case, I wonder if there is a common solution.