Add special "conservative" highlighting for style attribute values

[Monkatraz]

Was part of my old PR. Quoting myself:

This adds a form of syntax highlighting inside of CSS style attributes as requested by #381. This also adds a slight "fix" to attribute value number strings, which allows them to be quoted (pictures below).

Syntax highlighting is done "conservatively", as in the actual source.css grammar is not included. Instead, I wrote an "adequate" grammar, which should look fairly similar to how CSS normally looks in themes and is plenty good enough for Svelte's purposes. The primary reason I did it this way was to avoid any 'break-out' bugs. This addition, therefore, should actually avoid those bugs - however, because we don't really have a good test suite yet, I haven't been able to test this thoroughly.

What is actually highlighted in the grammar is very minimal - selectors, media blocks, etc. are not handled because, afaik, they won't ever appear in a style string. Additionally, the highlighting won't actually "check" for valid expressions (forgetting a semi-colon) because it doesn't parse the CSS using begin-end blocks. It uses match pretty much exclusively except for functions and strings.

Due to the lack of a test suite, I'd recommend caution when merging this - I can't see how my additions can go too wrong, but I don't know for sure. Textmate grammar definitions are cursed.

One issue I do know of is that any themes that specifically handle source.css may highlight interpolated Svelte expressions slightly wrong - e.g. in my theme, CSS variables are red, so Svelte variables become red inside the interpolated expression. This is a tradeoff, as I'd have to make things either very messy or complex to fix this. This isn't too common in themes.

Some screenshots:

My theme

Dark+

Light+

This extends/requires #1315. That PR should be merged first, or this one merged and that one closed.

[dummdidumm]

Thanks! I'm not 100% sure if I would want this as a user. That amount of different highlighting inline of an attribute could get confusing to me. I'll wait a little to give others the possibility to comment.

[blaumeise20]

I really like this feature, but it seems like you removed the orange quotes, right?