Deprecate Char-to-Number conversions in stdlib (JVM and JS)

- Int.toChar was left non-deprecated because the replacement is not intrinsic yet.
- Number.toChar was left non-deprecated because otherwise the deprecation propagates to the override, Int.toChar.

KT-23451

libraries/stdlib/src/kotlin/CharCode.kt

@@ -45,4 +45,5 @@ public expect fun Char(code: UShort): Char
 @SinceKotlin("1.5")
 @WasExperimental(ExperimentalStdlibApi::class)
 @kotlin.internal.InlineOnly
+@Suppress("DEPRECATION")
 public inline val Char.code: Int get() = this.toInt()