Extract delegate for TrixEditorElement

[seanpdoyle]

Related to #1023

In preparation for #1128, this commit introduces a module-private
Delegate class to serve as a representation of what form integration
requires for the <trix-editor> custom element. The structure of the
Delegate class mirrors that of the TrixEditorElement from which its
contents are extracted.

First, there are the properties that mimic those of most form controls,
including:

• labels
• form
• name
• value
• defaultValue
• type

With the exception of labels, property access is mostly proxied
through the associated <input type="hidden"> element (accessed through
its own inputElement property).

Next, the Delegate defines methods that correspond to the Custom
Element lifecycle events, including:

• connectedCallback
• disconnectedCallback
• setFormValue

The connected and disconnected callbacks mirror that of the
TrixEditorElement itself. These callbacks attach and remove event
listeners for click and reset events.

The setFormValue is named to correspond with
ElementInternals.setFormValue. Along with introducing this callback
method, this commit renames the TrixEditorElement.setInputElementValue
method to TrixEditorElement.setFormValue.

In addition to renaming setInputElementValue, this commit also defines
TrixEditorElement.formResetCallback (along with other empty
callbacks), then implements TrixEditorElement.reset as an alias. The
name mirrors the ElementInternals.formResetCallback.