Use heredoc/squish for inline css styles in oembed serializer

[mjankowski]

Noticed this while looking at #31990

The main change here is to pull these out to a heredoc'd/squish'd arrangement to make for slightly easier "css editing" if/when these need to be changed. Tried to improve the hash keys a bit while in there.

As a fun side project, I discovered some what I think is broken CSS/heredoc/hash syntax highlighting bugs in my editor, which I'll report to the relevant local authorities.

[ClearlyClaire]

As a fun side project, I discovered some what I think is broken CSS/heredoc/hash syntax highlighting bugs in my editor, which I'll report to the relevant local authorities.

Does it actually detect these as CSS? If not, would a heredoc with name CSS instead of STYLES help?

[mjankowski]

It should detect the heredoc contents as CSS when the start marker includes CSS, but seems slightly broken. That said, I'd rather not do a followup PR just to rename a marker ... so pushed a change here to use CSS, and will attempt to sort out the editor stuff separately.