Fully Undoable command stacks

What's Changed

• Buffer.evaluate { ... } is a new Result Builder block-based entry point
• undo() and redo() restore selection state in both directions if required: direct modifications don't alter the selection state, and direct user-based selection also is not undoable, but if you run a command group through Buffer.evaluate { ... } that's undoable, you'll get complete context restoration including selection for free
• CI: Run tests via GitHub Actions by @ikelax in #5

New Contributors

• @ikelax made their first contribution in #5

Full Changelog: 0.1.2...0.3.0

First Walking Skeleton

Comes with all the basics:

• Undoable<B> buffers to support undo/redo programmatically,
• NSTextViewBuffer to adapt NSTextView on macOS
• Modifying to group blocks of commands like
  • Insert to add text
  • Delete to remove text
• Select to change the selection/insertion point location