Add readability manual check?

Problem/Motivation

Summary of Slack conversation:

@Shaal pointed to the functionality at Readable, and wonders if some sort of inline hinting might be possible for readibility.

Proposed resolution

@Rkoller (and I) are skeptical of page-level readability scoring -- e.g, as a panel tool with the Outline and Alt text.

But Sa11y has readability logic ready to go. It would not be too difficult to add a test that flagged paragraphs with very long sentences. @Rkoller suggested using word count rather than reading score.

Remaining tasks

Determine feasibility of a manual check or panel tab. Note that this module is sponsored by Higher Ed, and Higher Ed really likes long sentences.

If there was a page-level alert for a very high reading reading level, it is not clear where that would be placed. On the worst paragraph?

User interface changes

Possible new panel tab.

API changes

Possible new test.