Adding ability to write custom checks + 6 new rules

Release notes: https://github.com/SonarQubeCommunity/sonar-css/milestone/10?closed=1

• Now based on strongly typed AST
• Ability to write custom checks (see plugin example and detailed explanations at https://github.com/racodond/sonar-css-custom-rules-plugin)
• Minfieds CSS are automatically excluded from the analysis
• New rule: Experimental selector combinators should not be used
• New rule: Obsolete pseudo-elements and pseudo-classes should not be used
• New rule: Experimental pseudo-elements and pseudo-classes should not be used
• New rule: Unknown pseudo-elements and pseudo-classes should not be used
• New rule: Experimental identifiers should not be used
• New rule: Case-insensitive flag should not be used

And many other improvements.