Initial Kotlin Multiplatform setup

[aSemy]

Initial groundwork for Kotlin Multiplatform #192

Depends on #194

I was expect this to be a lot more difficult! I indended just to do one module, but I found that they were all very easy to migrate. html-parser was the most involved.

That said, I can't run most of the tests (I'm on Windows), so I could have broken some stuff. And the really hard work of actually implementing JS and/or Native code can be done later.

WIP

• [x] Migrate test-utils
• [x] Update Kover config (or disable Kover if this is too difficult)
• [x] ~Configure Maven publishing buildSrc plugin (shouldn't be too much work to do, I can copy & paste some existing config that works)~ I've briefly tested this locally and it seems to work as expected.
• [ ] Verify that the new publications are correct and work. This means checking the POMs are correct and expose the right API dependencies.
• [x] ~jsExecution feature variant - I can't find an alternative for this with Kotlin Multiplatform. https://youtrack.jetbrains.com/issue/KT-33432. ~ I've simply added the 'maven publishing' config to the browser-fetcher project. I think that will achieve the same result.

Notes

• bump Kotlin to 1.7.10, and the language level to 1.7, and - ⚠ breaking change - the api level to 1.5 (from 1.4). Kotlin 1.7 has some nice improvements for Kotlin Multiplatform. And level 1.4 is deprecated. This seemed like a good time to bump it.
• JVM only
• All tests are still JUnit
• I didn't try migrating any code, just moving things into the correct source sets
• The real work was creating expect/actual definitions - so check them out and see if they make sense. The expect definitions are essentially like interfaces that the platform code will implement.
• The 'JS browser execution' feature probably won't work - I disabled the Gradle option for it
• The HttpFetcher and BrowserFetcher objects are pretty redundant, as they don't significantly extend from the BlockingFetcher interface. I think you can refactor the common code to only rely on the interface.
• Publishing and releasing are still TODO

[aSemy]

I've picked this back up again, but I've come across an issue with Kover. 0.5 had a bug https://github.com/Kotlin/kotlinx-kover/issues/183, and now 0.6 does too https://github.com/Kotlin/kotlinx-kover/issues/221

[christian-draeger]

Hey awesome news you proceed working on the PR 🙏🚀 If kover is blocking the further development I think we can just proceed while waiting for a new kover release that hopefully includes a fix.

Is kover / make the gradle plugins work the last thing that is missing to finish the PR?

And big thanks again for your contribution 🔥 I really appreciate.

[aSemy]

Apologies for the delay, and thanks for the nudge! :) I'll try and get this ready-for-review in the next couple of hours.

I've got an idea of how to fix Kover, so I'll give that a go. I've updated the main description with the TODOs.