Support for superscript and subscript

[ttrout55]

Pdfmake recently added support for superscript and subscript on text. It is implemented as just a boolean on the text object. can txt and Itxt be updated to have these keys available? I dont see it on thier documentation but here is the PR that made the change https://github.com/bpampuch/pdfmake/pull/2128/files

[Lugriz]

Hi @ttrout55! Thanks for sharing this new feature, it will be taken for next versions.