Fix case-insensitive character-wise comparison KT-45496

- Step 1: add failing tests
- Step 2: fix common case insensitive Char.equals
- Step 3: fix case insensitive String.equals in K/JS
- Step 4: enable unicode case folding in K/JS Regexes and
  string replacement (KT-45928)
- Step 5: fix case insensitive char comparison in K/N in String functions
  String.replace, equals, compareTo with ignoreCase

libraries/stdlib/src/kotlin/text/Char.kt

@@ -230,9 +230,10 @@ public fun Char.equals(other: Char, ignoreCase: Boolean = false): Boolean {
     if (this == other) return true
     if (!ignoreCase) return false
 
-    if (this.uppercaseChar() == other.uppercaseChar()) return true
-    if (this.lowercaseChar() == other.lowercaseChar()) return true
-    return false
+    val thisUpper = this.uppercaseChar()
+    val otherUpper = other.uppercaseChar()
+
+    return thisUpper == otherUpper || thisUpper.lowercaseChar() == otherUpper.lowercaseChar()
 }
 
 /**