Make tags and tag suggestions able to be styled by add-ons

[glutanimate]

I would like to extend Colorful Tags with the ability to also render user-selected tag colors in the editor. Right now this is not really possible as there is no stable & performant way to target individual tags and suggestion components.

This PR introduces two new data attributes, data-suggestion and data-tag-name that are applied to the corresponding elements, making them very easy to target via CSS or JS.

A couple of thoughts:

• I was thinking of perhaps renaming the attributes to use a common data-addon prefix, e.g. data-addon-tag and data-addon-suggestion. That might be a more expressive way of highlighting that these are consumed by add-ons while also making it easier to grep for add-ons that might be accessing the attributes in the future. WDYT?
• Do you think it would be prudent to escape some characters in the tag content? I wasn't able to find any Svelte-specific recommendations here. Given that Svelte seems to render the attributes double-quoted, it seems like replacing potentially ambiguous & and " would be the spec-conformant way. However, it doesn't look like we're doing this anywhere else and in some places we already are setting some attributes to values that could contain both " and & (e.g. button.title on Tag.svelte).

[dae]

• data-addon-* seems reasonable, but I don't have a strong preference
• I'm guessing that because Svelte sets the attributes via Javascript instead of raw HTML, ampersands/etc probably won't break, so I don't have much of a preference here either.

[glutanimate]

Cheers, I switched to data-addon-* and, having confirmed that & and " work, will leave the attribute handling as is then to remain consistent with other code.