Try/Catch support for native JS exceptions (#8955)

* Try/Catch support for native JS exceptions

* Better tests

tests/js/tnativeexc.nim

@@ -0,0 +1,31 @@
+discard """
+  action: "run"
+"""
+
+import jsffi
+
+# Can catch JS exceptions
+try:
+  asm """throw new Error('a new error');"""
+except JsError as e:
+  doAssert e.message == "a new error"
+except:
+  doAssert false
+
+# Can distinguish different exceptions
+try:
+  asm """JSON.parse(';;');"""
+except JsEvalError:
+  doAssert false
+except JsSyntaxError as se:
+  doAssert se.message == "Unexpected token ; in JSON at position 0"
+except JsError as e:
+  doAssert false
+
+# Can catch parent exception
+try:
+  asm """throw new SyntaxError();"""
+except JsError as e:
+  discard
+except:
+  doAssert false