fix(markdown): convert inline CSS from Google Docs to Markdown

[domcleal]

Summary

Extends the HTML to Markdown conversion to better support bold and italic formatting from Google Docs, which generates inline styles on a span element instead of strong/b/em/i type elements.

This makes it possible to copy limited formatting from Google Docs into the Markdown editor.

Test plan

1. Create a Google Docs document with bold, italic, bold & italic, and a bold & italic link.
2. Open the Posts collection in the test backend
3. Paste into the body and check the formatting matches

Example - copying formatted text from Google Docs into the Markdown editor and it's displaying correctly on the preview:

Checklist

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[demshy]

Tested and it works like a charm.

I was going to say that adding some tests would ensure that this feature will transfer to the upcoming Plate editor implementation, but I can see that they already have this covered https://platejs.org/