misc/hashcards: Add new port

Hashcards is a plain text-based spaced repetition system.

Features:

  - Plain Text: all your flashcards are stored as plain text files, so you can
    operate on them with standard tools, write with your editor of choice, and
    track changes in a VCS.
  - Content Addressable: cards are identified by the hash of their text. This
    means a card's progress is reset when the card is edited.
  - Low Friction: you create flashcards by typing into a text file, using a
    lightweight notation to denote flashcard sides and cloze deletions.
  - Simple: the only card types are front-back and cloze cards. More complex
    workflows (e.g.: Anki-style note types, card templates, automation) be can
    implemented using a Makefile and some scripts.
  - Efficient: uses FSRS for scheduling reviews, maximizing learning while
    minimizing time spent reviewing.

https://github.com/eudoxia0/hashcards/