Add support for css prop

[vpicone]

This was discussed in #48 but now that the css prop is here via v4, could we add support?

[eddiewang]

love this plugin. would love to see this supported as well :)

[vpicone]

I would add this myself, but can't make heads or tails of the regex

[KamaKAzii]

Also would love to see this supported! 👍

[njdancer]

Have made an update that supports highlighting of template strings inside css prop. Would appreciate any support available in getting double quoted strings working as well.

Here's a build for anyone who'd like to test this in the meantime.
https://github.com/njdancer/vscode-styled-components/releases/edit/0.0.26.pre1

[cahbb]

How far is this? Is it merged and live?