KTOR-5085: Add multiplatform support for XML serialization

[osipxd]

Subsystem ktor-serialization-kotlinx-xml

Motivation https://youtrack.jetbrains.com/issue/KTOR-5085/Add-multiplatform-support-for-XML

Solution Moved sources and some tests from jvm to common source set.

Docs change: https://github.com/ktorio/ktor-documentation/pull/513

[e5l]

@marychatte, could you check?

[osipxd]

@marychatte, thank you for your review!

1. gradle build fails

Unfortunately, I have only Linux x64 machine at the moment and all apple targets are ignored, so I cannot reproduce this problem.

2. There is also falling test XmlSerializationTest

Reproduced and fixed. Also, found out that [js, node] tests were failing because of this issue, so I disabled them for this module.

Here is the correct diff link to review the changes.