Mastodon extended the list of supported HTML elements in 4.2.0, but
didn't mention it in the release notes.

https://docs.joinmastodon.org/spec/activitypub/#sanitization) has the
list of supported elements.

Most of them are handled by Android, but a few are not, or are handled
badly. They are:

- `code`
- `pre`
- `ol` / `li`
- `ul` / `li`

Provide a `MastodonTagHandler` that handles these, by creating appropriate
spans in the content.

In particular, a new `LeadingMarginWithTextSpan` is used to create spans
with a leading margin that can contain text -- typically either bullets
for `ul` or numbers for `ol`.

The margin is sized to allow space for an `li` labeled `99. `, which
should hopefully be enough for use on Mastodon and similar servers. The
labels respect LTR and RTL locales, but do not allow setting custom labels
(e.g., through the `type` or `start` attributes on the content) because
Android's tag handler system does not pass the attributes down.

This duplicates some of the code in the pre-existing `PachliTagHandler`,
so move that code to `core.ui.taghandler` and adjust as necessary.

Fixes pachli#2217