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

core/builtins/native/kotlin/Number.kt

@@ -43,6 +43,7 @@ public abstract class Number {
     /**
      * Returns the [Char] with the numeric value equal to this number, truncated to 16 bits if appropriate.
      */
+//    @Deprecated("Direct conversion to Char is deprecated. Use toInt().toChar() or Char constructor instead.", ReplaceWith("this.toInt().toChar()"))
     public abstract fun toChar(): Char
 
     /**