Use new JSON unquoted literal in UnknownPolymorphicSerializer

[Whathecode]

kotlinx.serialization 1.5.0 introduced the ability to output unquoted literals:

In this release, there is a new addition to this API: JsonUnquotedLiteral constructor function.
It allows to produce a string that is not quoted in the Json output. This function has a lot of valuable
applications: from writing unsigned or large numbers to embedding whole Json documents without the need for re-parsing.
For an example, read the Encoding literal Json content docs.

It sounds like this can potentially be used to support an implementation of UnknownPolymorphicSerializer which doesn't rely on hacking into serializer's internals.