[WIP] Use yarn instead of npm

[mlms13]

Background

Hopefully the library is still in pretty decent shape, but there seems to be a little bitrot in some of the CI scripts. I'm hoping to clean up our build and release pipelines and get CI passing again (probably with some updated dependency versions and updated tools).

• [x] Switch from npm to yarn for local building (this will set us up for potentially using yarn workspaces)
• [x] Close the now-deprecated dependabot PRs
• [x] Bump dependency versions as needed
• [x] Bump the Node version in CI
• [x] Figure out why the ci.yml script is failing
• [x] Fix it

Notable Changes

• The location of the bisect_ppx-provided jest.js changed, so I updated it in our package.json
• One of our IO tests was checking the message of an OCaml-constructed exception, and it seems the runtime representation of those exceptions changed (although I didn't bump the bsb version, so I'm a little confused). Anyway, I skipped the test for now
• I chose not to take dependabot's suggestion to bump our transient dependency on marked to v2.0. This is a major change that may be incompatible with Docsify, and because we're just dealing with static markdown content, we're unlikely to be affected.

Not sure if I'll get to it here, but there are some additional things I'd like to do:

• [ ] Build with Melange in CI
• [ ] Fix doc publishing via the release.yml script
• [ ] Look into automatically publishing releases to npm

[mlms13]

I know this is unreviewed, but I think I'm going to merge it. Should be pretty safe, and we can always revert if anyone objects.