Fix handling non-ES5 identifiers in JS

When a Kotlin identifiers contains non-ES5 chars, JS BE
either reports error (for published declarations) or mangles name
(for non-published ones). The old approach relied on wrong
assuption that Java identifier = ES identifier.
However, that's not true. This commit introduces functions that
check identifiers according to ES5.1 spec rather than
using Character.isIdentifierStart[Part]

See KT-17476

js/js.translator/testData/box/jsName/peculiarIdentifiers.kt

@@ -23,9 +23,9 @@ fun test3(): String {
 }
 
 fun test4(): String {
-    val `()` = "OK"
-    fun `[]`() = `()`
-    return `[]`()
+    val `1(¢)` = "OK"
+    fun `[£]`() = `1(¢)`
+    return `[£]`()
 }
 
 fun box(): String {