Add first class Javascript/Typescript support to the Mill build tool

[monyedavid]

This pr implements the examples for jslib/dependencies.

#3927

Checklist:

• example/jslib/testing
  • 3-integration-suite-cypress
  • 3-integration-suite-playwright

[lihaoyi]

I think at a high level, this example needs to be significantly shortened. Let's trim all the CSS, robots.txt, logos, all the boilerplate comments from the .html files: just leave a JS server, JS client, and the integration tests.

This example doesn't need to be realistic, since there are other examples in the javascriptlib/ folder that show the other parts, so we can just focus on the core of how the integration tests are wired up and written

[monyedavid]

@lihaoyi are you aware mill can no longer handle special characters?

when matching: typescript/src/server/playwright/app.test.ts:4:9 › React App › displays the heading...
the › amongst others will throw an error causing a failed matched. I am positive at some point that would work as expected.

-- Edit:
Not sure if its a mill problem, but it works fine in local environment

[lihaoyi]

I think this looks good, thanks!