[frontend][backend] create shared types for those used across frontend and backend

[zxt-tzx]

Currently building out backend and it seems to me that the DTOs are shared types across frontend and backend and that we should put them in a shared folder like in AskGov.

(actually not entirely sure about this, since the DTOs on the backend uses decorators, but not those on the frontend hmm)

Wonder if this could be more general (i.e. not just DTOs, but other types too)

[zxt-tzx]

Might be useful to refer to opengovsg/askgovsg#216

Related TODOs added in #293

[zxt-tzx]

Look into how nemsw does it (apparently using turborepo).

UPDATE 1: also relevant: https://www.robinwieruch.de/javascript-monorepos/

UPDATE 2: aborted; not using Turborepo after all, see #376 (comment)

[zxt-tzx]

More TODOs added in #308