Replace Makefile with turborepo

[timostamm]

This repository is using a Makefile because it initially used Go for code generation.

We have most tools available as npm packages by now, and in connectrpc/connect-playwright-es, we switched to a new turborepo setup we want to standardize on.

The plan is to switch this repository to the same setup, which should give us a decent speedup in CI and local development, and also make it easier for contributors to get started.

There are two remaining complications:

• Conformance tests are currently run against Docker servers, and we need to make sure they are running during tests.
• We are running some (but not all) test suites on several versions of Node.js. The best way forward is likely to use the setup-node action with a matrix in the CI workflow instead.

[timostamm]

The two complications are eliminated with #1103 and #1110.

There's a (small) new one with #1079 (review). Fixed by #1111.