Map<Int, V> and other non-string-key maps aren't decoded

[devcrocod]

JsonMapDecoder assumes map keys are strings. Maps with non-string primitive keys (like Map<Int, V>), which kotlinx.serialization supports by encoding the key as a string and converting back, aren't handled.

See JsonMapDecoder.kt.

Needs key-type conversion added, matching how kotlinx.serialization's other JSON formats handle non-string map keys.