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/ttempgen.nim

@@ -0,0 +1,79 @@
+discard """
+  output: '''
+foo
+'''
+"""
+
+block: # #12672
+  var a = @[1]
+  let i = 1
+  inc a[i-1]
+
+  var b: seq[int]
+  doAssertRaises(IndexDefect): inc b[0]
+  doAssertRaises(IndexDefect): inc b[i-1]
+
+  var x: seq[seq[int]]
+  doAssertRaises(IndexDefect): # not TypeError
+    inc x[0][i-1]
+
+block: # #14087
+  type Obj = object
+    str: string
+
+  var s = @[Obj(str: "abc"), Obj(str: "def")]
+  s[1].str.add("ghi")
+  s[s.len - 1].str.add("jkl")
+  s[^1].str.add("mno")
+  s[s.high].str.add("pqr")
+
+  let slen = s.len
+  s[slen - 1].str.add("stu")
+
+  let shigh = s.high
+  s[shigh].str.add("vwx")
+
+  proc foo(): int =
+    echo "foo"
+    shigh
+  s[foo()].str.add("yz")
+  doAssert s[1].str == "defghijklmnopqrstuvwxyz"
+
+block: # #14117
+  type
+    A = object
+      case kind: bool
+      of true:
+        sons: seq[int]
+      else: discard
+
+  var a = A(kind: true)
+  doAssert a.sons.len == 0
+  a.sons.add(1)
+  doAssert a.sons.len == 1
+
+import tables
+
+block: # #13966
+  var t: Table[int8, array[int8, seq[tuple[]]]]
+
+  t[0] = default(array[int8, seq[tuple[]]])
+  t[0][0].add ()
+
+block: # #11783
+  proc fun(): string =
+    discard
+
+  var ret: string
+  ret.add fun()
+  doAssert ret == ""
+
+block: # #12256
+  var x: bool
+
+  doAssert x == false
+
+  reset x
+
+  doAssert x == false
+  doAssert x != true