[js] add testcase for array indexDefect and remove todo (#19838)

* remove unused opcSubstr

* [js] add testcase for array indexDefect

* Revert "remove unused opcSubstr"

This reverts commit cb461f2545234d62c1e0b83318f3e5495c97de52.

lib/system/jssys.nim

@@ -731,10 +731,6 @@ proc parseFloatNative(a: string): float =
   `result` = Number(`a2`);
   """
 
-#[
-xxx how come code like this doesn't give IndexDefect ?
-let z = s[10000] == 'a'
-]#
 proc nimParseBiggestFloat(s: string, number: var BiggestFloat, start: int): int {.compilerproc.} =
   var sign: bool
   var i = start

tests/js/tindexdefect.nim

@@ -0,0 +1,9 @@
+discard """
+  outputsub: "unhandled exception: index 10000 not in 0 .. 0 [IndexDefect]"
+  exitcode: 1
+  joinable: false
+"""
+
+var s = ['a']
+let z = s[10000] == 'a'
+echo z