Create a document query and verification tool

[matthew-carroll]

Editor-level tests require verifying various document and editor properties. This is very verbose when we use the standard tools for constructing such information.

Investigate what a query and verification tool might look like to simplify tests.

Verbose selection verification example:

expect(
  _editContext.composer.selection,
  equals(
    const DocumentSelection.collapsed(
      position: DocumentPosition(
        nodeId: '1',
        nodePosition: TextNodePosition(offset: 11),
      ),
    ),
  ),
 );

A possible alternative representation:

expect(_editContext, hasTextSelection("1", 11));

Verbose document creation:

MutableDocument(
              nodes: [
                ParagraphNode(
                  id: '1',
                  text: AttributedText(text: 'Text with [DELETEME] selection'),
                ),
              ],
            )

A more concise document creation:

// Notice the "**", we can auto-parse as Markdown to avoid the very verbose span markers.
DocBuilder()
  ..paragraph(id: '1', "Text with **[DELETEME]** selection");