KT-49721 Avoid matching from the same position after a zero-width match

In JS, RegExp can return a match before the specified start index,
if it has matched at that index, but it is in the middle of a surrogate
pair. Account for that when advancing to the next position after
a zero-width match so that it doesn't get to the middle of SP.

libraries/stdlib/js/test/core/testUtils.kt

@@ -28,4 +28,6 @@ public actual val isFloat32RangeEnforced: Boolean = false
 actual val supportsSuppressedExceptions: Boolean get() = true
 
 // TODO: implement named group reference in replacement expression
-public actual val supportsNamedCapturingGroup: Boolean get() = false
+public actual val supportsNamedCapturingGroup: Boolean get() = false
+
+public actual val regexSplitUnicodeCodePointHandling: Boolean get() = true