Give definitions of technical terms

[jotaen]

Resolves #19.

• Introduce appendix with a glossary of technical terms.
• Replace “whitespace” with “blank character”, because whitespace includes vertical whitespace such as newlines.
• The Unicode Punctuation category is quite broad (see here for its subcategories), which is probably not necessary. The important thing is to not limit the ability for tags or due dates to appear as words in the natural flow of sentences, e.g.:
  • [ ] Do taxes (#work, #finances)
  • [ ] Call #Mike: discuss weekend plans
  • [ ] Paint fence -> 2022-04-18, Larry comes over to help
  • [ ] Change tires (-> 2022-05-02)