Add jsheaders

[juancarlospaco]

• Added jsheaders module for Headers for the JavaScript target. See https://github.com/nim-lang/Nim/pull/16339#issue-538769691 and https://github.com/nim-lang/Nim/pull/16339#issuecomment-744485709 and https://github.com/nim-lang/fusion/pull/57#issue-539766026 and https://github.com/nim-lang/fusion/pull/56#issue-539747803 and https://github.com/nim-lang/fusion/pull/55#issue-539726540 and https://github.com/nim-lang/fusion/pull/58#issue-539886249

[timotheecour]

@juancarlospaco ok, can you find another reviewer to get a 2nd LGTM so we can merge this?