feat(js): jsutils/consts from_js_const

Author: litlighilit

src/pylib/jsutils/consts.nim

@@ -0,0 +1,13 @@
+
+const
+  InNodeJs = defined(nodejs)
+import std/jsffi
+when InNodeJs:
+  let econsts = require("constants")
+else:
+  let econsts{.importjs: """(await import("node:constants")).errno""".}: JsObject
+
+template from_js_const*(name; defval: int): int =
+  bind econsts, isUndefined, to, `[]`
+  let n = econsts[astToStr(name)]
+  if n.isUndefined: defVal else: n.to(int)