🐛 `MarkdownViewer` incorrectly parses task items in `1. - [ ]` format

[iansan5653]

This issue was originally reported as https://github.com/github/memex/issues/11844

When a Markdown task list has items delimited with both a number and a dash or asterisk, the MarkdownViewer component fails to recognize the items and thus incorrectly checks the wrong list items:

https://user-images.githubusercontent.com/1062491/188623750-56d3a607-b7f5-4b2f-b8be-233a85e96bc5.mov

This is a draft component and a pretty uncommon edge case so I wouldn't prioritize this at the moment unless we start seeing more reports.

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The problem is the list-item parsing regular expression:

https://github.com/primer/react/blob/679e699d2fa864a505251e07b0c2dd19ed4ad231/src/drafts/MarkdownEditor/_useListEditing.ts#L32

The ([*-]|(\d+)\.) group assumes that a list item will start with a bullet (- or *) or a number (1.). These list items start with a number and a bullet. I'm surprised the Markdown pipeline renders this list at all, but because it does, we should probably handle this case.

[iansan5653]

A note from our FR from the original issue:

I couldn't replicate this outside of the non-standard edgecase of 1. - [ ] prefixed items, which seemed unconventional.

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