[ci] Add turborepo for faster builds

[styfle]

yarn build (first time, no cache)

• Before: 2m10s
• Turbo: 1m23s

yarn build (second time, with cache)

• Before: 25.64s
• Turbo: 0.59s

This is largely due to concurrency (previously we ran each build sequentially)

We can improve build times even more once we implement remote caching.

Caveats

I had to add an explicit dependency on @vercel/build-utils even though the builders/runtimes don't actually need it but this made the turborepo config much simpler. A side-effect of this will be each time build-utils changes, we'll need to publish go/node/python/ruby, but thats fine because we would have already needed to publish the CLI too.

Docs:

• https://turborepo.org
• https://turborepo.org/docs

[codecov]

Codecov Report

Merging #7193 (8457962) into main (7039771) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #7193   +/-   ##
=======================================
  Coverage   49.17%   49.17%           
=======================================
  Files         135      135           
  Lines        5426     5426           
  Branches     1358     1358           
=======================================
  Hits         2668     2668           
  Misses       2745     2745           
  Partials       13       13           

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