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/test/text/CharTest.kt

@@ -205,7 +205,7 @@ class CharTest {
     @Test
     fun charCategory() {
         for ((char, categoryCode) in charToCategory()) {
-            assertEquals(categoryCode, char.category.code, "char code: ${char.toInt().toString(radix = 16)}")
+            assertEquals(categoryCode, char.category.code, "char code: ${char.code.toString(radix = 16)}")
         }
     }