Clickable text support

[breville]

This adds support for clickable text.

When the text markdown contains:

[text](#clickable=id)

This code will generate:

<b data-id="id" class="clickable-text">text</b>

The main repo will make use of this functionality in code-dot-org/code-dot-org#55861.

[Hamms]

This looks great! Couple minor suggestions for improvement, but nothing blocking. And it always makes me happy to see initial implementations get follow-ups like this, thank you for the extra effort!