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/t14153.nim

@@ -0,0 +1,22 @@
+discard """
+  output: '''
+index 5 not in 0 .. 2
+index 5 not in 0 .. 2
+'''
+"""
+
+var x = @[1, 2, 3]
+
+try:
+  echo x[5]
+except IndexError:
+  echo getCurrentExceptionMsg()
+except:
+  doAssert false
+
+try:
+  x[5] = 8
+except IndexError:
+  echo getCurrentExceptionMsg()
+except:
+  doAssert false