Display informations on hover

For example, show descriptions and documentation links for:
- modifiers
- scriptlets
- Extended CSS pseudo classes

Also, we need to introduce an option within extension settings where this feature can be adjusted.

Reference: https://code.visualstudio.com/api/language-extensions/programmatic-language-features#show-hovers

Needed: https://github.com/AdguardTeam/AGLint/issues/192