many bugfixes for js (#14158)

* many bugfixes for js

fixes #12672, fixes #14153, closes #14123, closes #11331, fixes #11783, fixes #13966, fixes #14087, fixes #14117, closes #12256.

mostly fixes the fact that it was allowed to assign to newly created temp variables. additionally attempts to get rid of null initialized seqs/strings (though they might pop up here and there); this simplifies a lot of things and makes code size smaller. even if null seqs/strings pop up here and there it's still better than all those bugs existing.

* formatting fixes

* CI fixes

* more CI fixes

tests/js/t12672.nim

@@ -0,0 +1,12 @@
+discard """
+  output: ""
+"""
+
+proc foo =
+  var x: seq[seq[int]]
+  for row in x.mitems:
+    let i = 1
+    echo row
+    inc row[i-1]
+
+foo()