Add `selector-class-pattern` and `selector-id-pattern` rules

[Stephen Edgar]

https://make.wordpress.org/core/handbook/best-practices/coding-standards/css/#selectors

"Similar to the WordPress PHP Coding Standards for file names, use lowercase and separate words with hyphens when naming selectors. Avoid camelcase and underscores."

https://github.com/stylelint/stylelint/tree/master/src/rules/selector-class-pattern
https://github.com/stylelint/stylelint/tree/master/src/rules/selector-id-pattern