[PassageMarkdown] Refactor RefStart/RefEnd into components
[PassageMarkdown] Refactor RefStart/RefEnd into components

Summary:
Refactor refStart and refEnd rules to output components, which
will simplify adding screenreader-only content to them in the
future.

Test Plan:
Make a passage and see that using a ref to calculate line numbers
still works.

Reviewers: michelle, emily

Reviewed By: emily

Subscribers: john, #a11y

Differential Revision: https://phabricator.khanacademy.org/D17791

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