Set up separate dev and prod builds with esbuild

[aomarks]

Note to reviewers: I first used tsc + Rollup in this PR, and then replaced it with esbuild, so you'll see those commits in the middle. You can ignore them, or read them if you're interested.

Sets up separate dev and prod eleventy builds for the new webcomponents.org. Both builds use esbuild.

Running npm run watch:dev or npm run watch:prod from the top-level will give you live refresh versions of either build.

The dev build is super fast because [1] it uses symlinks so that Eleventy doesn't need to rebuild when static assets or JS changes, and [2] it runs esbuild with no minification or bundling.

There is a separate check:types command in the client package which runs TypeScript in noEmit mode. This runs in parallel when you use either of the top-level watch:* commands. So the experience is that the browser updates immediately, and a few moments later you also get any type errors.

This design lets us use identical <script> tags in both dev and prod mode, which is nice. On lit.dev we needed to generate different script tags for each mode.

Also addresses the post-merge comments from #1333

[justinfagnani]

should this have a dependency on ../client:build:dev?

[aomarks]

No, because otherwise every time the client code changes, Eleventy will re-run unnecessarily, because in dev mode that's just a symlink.

[justinfagnani]

Hmm... seems like that leaves a situation where the client output could be stale. Is there a different type of dependency at play here. Like: when you run task A, ensure B is up to date, but when you run B, do not dirty A.

[aomarks]

Yeah, that's what google/wireit#238 would be -- but I got a lot of feedback that the API and naming is weird, so I've been sitting on that for ages. Maybe need to get back to it.