standard keyboard shortcuts

[wandering-logic]

trying to put together the most intuitive keyboard shortcuts. Where there's consensus below we should adopt. Where there's divergence we should pick at least one of these (not invent our own).

wikipedia summary table: https://en.wikipedia.org/wiki/Table_of_keyboard_shortcuts
google docs: https://support.google.com/docs/answer/179738

onenote pwa: https://support.microsoft.com/en-us/office/keyboard-shortcuts-in-onenote-44b8b3f4-c274-4bcc-a089-e80fdcc87950#picktab=web

github: https://docs.github.com/en/get-started/accessibility/keyboard-shortcuts

slack: https://slack.com/help/articles/201374536-Slack-keyboard-shortcuts#windows-linux-1

[wandering-logic]

see also issue #25 where we discuss allowing certain markdown-like (non-control) characters to be recognized as shortcuts.