Bugfix/add char to platformstable types

[hughlunnon]

issue documented here - #3533 -

the block in kotest-common/src/commonMain/kotlin/io/kotest/mpp/stable.kt

   String::class,
   Int::class,
   Long::class,
   Double::class,
   Float::class,
   Byte::class,
   Short::class,
   Boolean::class,
   Pair::class,
   Triple::class,
)

missed "Char", which means the code

    context("test something") {
        data class TestData(val input: String?, val expected: Char)
        withData(
            TestData("US", 'Y'),
            TestData("UK", 'N'),
            TestData(null, 'N'),
        ) { (input, expected) ->
           doSomething(input) shouldBe expected
        }
    }

gives lots of warnings like

Warning, type class kotlin.Char used in data testing does not have a stable toString()

char was previously being detected as "unstable", is now correctly identified as stable