Convert Rest.js module and tests to TypeScript

[maier49]

Convert src/Rest.js to TypeScript as src/Rest.ts, and tests/unit/Rest.js as tests/unit/Rest.ts.

This should allow the unit tests in tests/unit/Rest to pass again. In order to run unit tests with the changes, any instances of Rest will need to be changed to Rest.default in other JS modules until we convert them as well.

See #130 for general information on the conversion. If you have questions, comments, or thoughts for this module beyond the conversion itself, you can leave comments on this issue.

[sebilasse]

just a note: There is some more typescript in this fork of the @maier49 branch: https://github.com/sebilasse/dstore/tree/master/src – but my part is very experimental for now