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/trepr.nim

@@ -308,21 +308,21 @@ b = 0,
 c = 0.0,
 d = '\0',
 e = eA,
-f = nil,
+f = "",
 g = {},
 h = {},
-i = [nil, nil, nil],
-j = nil,
+i = ["", "", ""],
+j = @[],
 k = 0,
-l = [a = nil,
-b = nil],
+l = [a = "",
+b = @[]],
 m = nil,
 n = nil,
-o = [Field0 = [a = nil,
-b = nil],
-Field1 = nil],
+o = [Field0 = [a = "",
+b = @[]],
+Field1 = ""],
 p = nil,
-q = nil]
+q = ""]
 """)
   doAssert(repr(cc) == """
 [a = 12,