[editing] Enumerating CommandEvents

I’ve taken a stab at enumerating possible types for CommandEvents.

You can find the list at
 https://docs.google.com/spreadsheets/d/11Gdl1fnVrEmxiT5dDOnBAwjnog2Xc43qOmFuXfekmUo/edit?usp=sharing

This list contains:

   - execCommands from MDN (
   https://developer.mozilla.org/en-US/docs/Web/API/document.execCommand)
   - execCommands from the HTML Editing APIs spec (
   https://dvcs.w3.org/hg/editing/raw-file/tip/editing.html)
   - additional execCommands exposed by Blink/WebKit
   - execCommands Blink/WebKit use internally (included as a gauge of
   useful primitives for building editing libraries)

I’ve categorized each command (Formatting, Insertion, etc), and merged
related commands (aka, Indent/Outdent) to simplify the list a bit.  The
list is includes more events that *should* be exposed as CommandEvents.  To
start, we can eliminate selection and clipboard events, as they should be
covered by ClipboardEvents and selection events (included in spreadsheet
just for completeness as they have corresponding execCommands).

Is this useful as a starting point? What other areas should be added?
 Which commands should be removed?

Julie