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/test/text/CharTest.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2021 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -9,6 +9,16 @@ import kotlin.test.*
 
 class CharTest {
 
+    companion object {
+        val equalIgnoreCaseGroups = listOf(
+            "Aa", "Zz", "üÜ", "öÖ", "äÄ",
+            "KkK", "Ssſ", "µΜμ", "ÅåÅ",
+            "DŽDždž", "LJLjlj", "NJNjnj", "DZDzdz", "ͅΙιι", "Ββϐ", "Εεϵ",
+            "Κκϰ", "Ππϖ", "Ρρϱ", "Σςσ", "Φφϕ", "ΩωΩ", "Ṡṡẛ",
+            "Θθϑϴ", "Iiİı",
+        )
+    }
+
     @Test
     fun charFromIntCode() {
         val codes = listOf(48, 65, 122, 946, '+'.code, 'Ö'.code, 0xFFFC)
@@ -145,6 +155,20 @@ class CharTest {
         testFails(100, radix = 110)
     }
 
+    @Test
+    fun equalsIgnoreCase() {
+        val nonEqual = equalIgnoreCaseGroups.flatMap { allEqualChars ->
+            allEqualChars.flatMap { c1 -> allEqualChars.mapNotNull { c2 ->
+                    if (!c1.equals(c2, ignoreCase = true)) "$c1 != $c2" else null
+                }
+            }
+        }
+        if (nonEqual.isNotEmpty()) {
+            fail("Expected chars to be equal ignoring case:\n${nonEqual.joinToString("\n")}")
+        }
+    }
+
+
     private fun charToCategory() = mapOf(
         '\u0378' to "Cn",
         'A' to "Lu",    // \u0041