Use rewatch to build runtime, tests etc.

[cknitt]

Since #7063 and #7068, the infamous ninja.js is gone, and runtime/stdlib and tests are built with the standard ReScript build tooling (rescript.json, rescript wrapper script, bsb/bsb_helper/ninja).

In order to eventually get rid of bsb, bsb_helper and ninja completely, we need to build all these with rewatch instead:

• runtime
• tests/tests
• tests/build_tests/*
• tests/gentype_tests/*

Note:

• Building runtime is a bit special with -make-runtime and the resulting paths in the JS output. When I last tried it with rewatch, I wasn't able to get the same paths in the JS output.
• Even with the standard tooling, I was only able to get it to work as required with all source files in the runtime root. But the goal would be to be able to group them into subfolders (core, belt, js, primitives, ...).

@jfrolich @rolandpeelen could you take a stab at this?

[rolandpeelen]

@jfrolich -- Shall we have a chat about this the coming week / divide & conquer? I might be able to clear up some time to do some smaller tasks

[jfrolich]

Yes, same same! 👍