Add Kover support

[0xnm]

This PR adds Kover support declared in #3610.

Overall functionality is similar to what ScoverageModule and https://github.com/lefou/mill-jacoco are doing:

• Each test run gets instrumented by Kover and binary report is generated (this requires mixing in KoverTests trait)
• There is a possibility to generate HTML/XML report out of it using __.kover.htmlReport / __.kover.xmlReport respectively on each module having KoverModule mixed in.
• Also there is a possibility to declare module implementing KoverReportModule trait and use htmlReportAll / xmlReportAll tasks to collect coverage for all modules in the project.

Kover Agent documentation: https://kotlin.github.io/kotlinx-kover/jvm-agent
Kover Reporter documentation: https://kotlin.github.io/kotlinx-kover/cli/

[lihaoyi]

Looks good, thanks @0xnm ! Will transfer the bounty