add editor hook to listen for entered text

[bantic]

In order to implement something like a system where typing "@" triggers an editor event that a consumer can listen to and then later replace with an atom (to represent that @-mention in some semantic/metadata-y sense).

The sort of flexible input configuration that Mousetrap uses seems like a good path.

This system should handle:

• inserted text (if the user types "abc", or "@" e.g.) — callback should be invoked after the text is inserted
• keydown or keyup with and without modifiers ("command+k") — callback should be invoked on key down/up event and cancel default behavior

In the callback the following information should be available:

• cursor position (probably just read this from editor.range)
• have access to the editor
• be able to specify that other listeners for the same text/key-combo should fire (by default only the first listener would likely fire)

[bantic]

The current hooks the editor exposes for reacting to use key- and text- related events are:

• registerTextExpansion — these take required "text" and "trigger key" args (e.g.: text "*" and trigger <SPACE>). The callback (which is passed as the "run" key) is invoked when the trigger key is pressed after the "text" is entered at the start of the section.
• registerKeyCommand — these take a required "str" arg that is similar to Mousetrap style (e.g. "META+K"). The callback is invoked when the key-combination keyDown event happens

[bantic]

After thinking it over, there are two distinct (though related) concerns — reacting to text the user typed and a key-combination the user pressed. Specifying whether to trigger on key-up or -down only applies to key-combos, and options like atStart: true|false only make sense to text-insertions. In that light it makes sense to keep them as separate hooks.

To address the use case described in this issue I'm going to change the text expansion hooks in the editor to be more flexible. They can be updated to make the trigger optional, and to accept an atStart prop that determines whether they will fire any time or only when the text is at the start of the section.

E.g.:

// react to @-mentions
editor.registerTextExpansion({
  text: "@",
  // no explicit trigger. This expansion will trigger after user types "@" (but will leave the "@" text in the editor)
  atStart: false,  // any time the text is entered,
  run: callbackFn // this callback would likely delete the "@" text and replace it with a "mention" atom
});

// expand "*" at the start of a section into a list section (this is an existing built-in expansion)
editor.registerTextExpansion({
  text: "*",
  trigger: " ", // if the user types a " " after the "*", this expansion will be invoked — only the "*" text will be in the editor because the expansion will capture and cancel the keypress for the " " char
  atStart: true,
  run: callbackThatTogglesSectionToListItem
});

cc @rlivsey let me know if this makes sense to you

[rlivsey]

@bantic looks great to me!