Add js BigInts (#16409)

* Add BigInts
* Renames tos plurals
* Improve Stringifications
* Update changelog.md

Co-authored-by: flywind <43030857+xflywind@users.noreply.github.com>

* RunnableExamplerize
* discard the discardable pragma
* Several improvements from peer reviews, more docs
* More doc, more test
* More doc, more test
* Better error message 'Error: usage of low is an {.error.} defined at jsbigints.nim' instead of just 'type mismatch JsBigInt'
* is an overload, rename
* proc to scare kids away
* Update lib/js/jsbigints.nim

Co-authored-by: Timothee Cour <timothee.cour2@gmail.com>

* https://github.com/nim-lang/Nim/pull/16409#discussion_r554365041

Co-authored-by: flywind <43030857+xflywind@users.noreply.github.com>
Co-authored-by: Timothee Cour <timothee.cour2@gmail.com>

tools/kochdocs.nim

@@ -14,8 +14,11 @@ const
   webUploadOutput = "web/upload"
 
 var nimExe*: string
+const allowList = ["jsbigints.nim"]
 
-template isJsOnly(file: string): bool = file.isRelativeTo("lib/js")
+template isJsOnly(file: string): bool =
+  file.isRelativeTo("lib/js") or
+  file.extractFilename in allowList
 
 proc exe*(f: string): string =
   result = addFileExt(f, ExeExt)