Update AGTree to v2

[scripthunter7]

We need to update AGTree to v2. It is important to note that AGTree v2 no longer parses CSS directly, so we need to do that separately using ECSSTree to keep CSS validation in the linter.

I suggest adding linter rules for CSS.

To avoid possible re-parsing of CSS codes, I propose to introduce a store that all linter rules have access to, so instead of re-parsing, we only need to read the cache (since we put the CSS code there the first time we parse)