[backport 1.0] add backend support for js bigint (#16606)

* add backend support for js bigint

* cleanup

* add tests

* add -d:nimHasJsBigIntBackend

* cleanup

* more tests

(cherry picked from commit 025ca660f7)

compiler/condsyms.nim

@@ -117,3 +117,4 @@ proc initDefines*(symbols: StringTableRef) =
   defineSymbol("nimHasInvariant")
   defineSymbol("nimHasStacktraceMsgs")
   defineSymbol("nimHasStacktracesModule")
+  defineSymbol("nimHasJsBigIntBackend")

compiler/jsgen.nim

@@ -1591,7 +1591,10 @@ proc createVar(p: PProc, typ: PType, indirect: bool): Rope =
   var t = skipTypes(typ, abstractInst)
   case t.kind
   of tyInt..tyInt64, tyUInt..tyUInt64, tyEnum, tyChar:
-    result = putToSeq("0", indirect)
+    if $t.sym.loc.r == "bigint":
+      result = putToSeq("0n", indirect)
+    else:
+      result = putToSeq("0", indirect)
   of tyFloat..tyFloat128:
     result = putToSeq("0.0", indirect)
   of tyRange, tyGenericInst, tyAlias, tySink, tyOwned: