[Bug]: Failure to parse relative CSS rules
### What happened?

In FontEmbedder.ts of tldraw editor, `getCurrentDocumentFontFaces` will fail to fetch the href of a CSS rule if it is relative: https://github.com/tldraw/tldraw/blob/main/packages/editor/src/lib/exports/FontEmbedder.ts#L97

I noticed this while using the export to PNG/SVG functionality. When doing an export, the FontEmbedder crawls through document stylesheets, following any import rules. It does this by fetching the rule's href. Let's say once of these rules are relative, for instance `@import "../style.css"`. If you try to fetch that directly, you would be effectively doing `GET domain.com/style.css`, resulting in 404. Let's say the stylesheet containing the rule lives at `domain.com/a/b/parent_style.css`, then you'd want the rule's href to resolve to `domain.com/a/style.css`.

A workaround could be to make the href absolute first, so instead of: `fontFaces.push(fetchCssFontFaces(rule.href))`, you could do something along the lines of:

```
const absoluteHref = new URL(rule.href, rule.parentStyleSheet?.href ?? undefined).href
fontFaces.push(fetchCssFontFaces(absoluteHref))
```

### How can we reproduce the bug?

Make an SVG export while using a stylesheet with a relative import rule.

### What browsers are you seeing the problem on?

_No response_

### Where did this happen?

The developer package

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