Add jsform

[juancarlospaco]

See nim-lang/Nim#16339 (comment) and nim-lang/Nim#16339 (comment) and #57 (comment) and #56 (comment) and #55 (comment)
and #58 (comment)

[timotheecour]

how about:

func newFormData*(): FormData =
  when defined(nodejs):
    asm """
    var tmp = require("form-data")
    `result` = new tmp()
    """
  else:
    asm "`result` = new FormData()"

...

runnableExamples:
  from strutils import startsWith
  if defined(fusionJsFormdataTests):
    let data: FormData = newFormData()
    when defined(nodejs):
      data.add "key0", "value0"
      data.add "key1", "value1"
      var ret: cstring
      asm """`ret` = `data`.getHeaders()["content-type"]"""
      doAssert ($ret).startsWith """multipart/form-data; boundary=""", $ret
    else:
      data["key0".cstring] = "value0".cstring
      data.add("key1".cstring, "value1".cstring)
      data.delete("key1".cstring)
      doAssert data.hasKey("key0".cstring)
      doAssert data["key0".cstring] == "value0".cstring

tested with:

npm i form-data
nim doc -b:js --doccmd:'-d:fusionJsFormdataTests -d:nodejs' src/fusion/js/jsformdata.nim

note:
there's no set in that nodejs package apparently, see form-data/form-data#488

EDIT

ignore this comment, looks like require("form-data") is too different or at least this could be done in future work; IMO the better way would be to fix this: timotheecour/Nim#197 (in future work)

[ringabout]

LGTM after addressing the last comment