feat: resolve css variables

[paviad]

Current style sheet logic ignores CSS variables, and can break when parsing them.

This PR properly resolves CSS variables.

[denighte]

Hi @paviad, thank you very much for your idea and contribution, but unfortuantely we had to fix it differently than you did, since your code does not take into account a lot of cases. Also we decided to not use regex because of performance concerns. Here is some of the missed cases:

1. Shorthands resolving
2. Several var() expression in one css statement e.g. 1px var(--style) var(--color)
3. Complex defaults handling and neted vars, e.g. var(--test, var(--another, 'black'))
4. Cyclic references, e.g. --one: var(--two); --two: var(--one)
5. Validation handling, e.g. word-break: var(--incorrect) should log a message that css declaration can't be resolved
   And other more minor stuff
   You can see all the changes in the following commit: 75380e906f718e34613ff3d63da44ce0738cde7e