[closes #11625 and closes #2488]add global and threadvar(with --threads:off mode ) pragmas supports for JS backend (#15772)

* add global pragma support for js backend

* globalThis

* add support for threadvar

* more tests

* Update compiler/jsgen.nim

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

tests/js/tglobal.nim

@@ -0,0 +1,30 @@
+block global:
+  proc getState(): int =
+    var state0 {.global.}: int
+    inc state0
+    result = state0
+
+  for i in 0 ..< 3:
+    doAssert getState() == i + 1
+
+  for i in 0 ..< 3:
+    once:
+      doAssert i == 0
+
+
+block threadvar:
+  proc getThreadState0(): int =
+    var state0 {.threadvar.}: int
+    inc state0
+    result = state0
+
+  for i in 0 ..< 3:
+    doAssert getThreadState0() == i + 1
+
+  proc getThreadState1(): int =
+    var state1 {.threadvar.}: int
+    inc state1
+    result = state1
+
+  for i in 0 ..< 3:
+    doAssert getThreadState1() == i + 1