Correctly mangle this in the JS backend (#8853)

As shown in pragmagic/karax#67 using `this` as parameter name made the
codegen output wrong code (and the user didn't notice the errors in the
browser console).

tests/js/tthismangle.nim

@@ -0,0 +1,23 @@
+proc moo1(this: int) =
+  doAssert this == 42
+
+proc moo2(x: int) =
+  var this = x
+  doAssert this == 42
+
+proc moo3() =
+  for this in [1,1,1]:
+    doAssert this == 1
+
+proc moo4() =
+  type
+    X = object
+      this: int
+
+  var q = X(this: 42)
+  doAssert q.this == 42
+
+moo1(42)
+moo2(42)
+moo3()
+moo4()