Introduce CSS preprocessor linting to warn against use of inline-block

[cjcenizal]

This supersedes #9185.

Background

Using display: inline-block introduces subtle UI problems due to the inline-block spacing problem. Let's introduce a CSS preprocessor linter to warn developers against using this property unless absolutely necessary. This will encourage us to use alternatives, such as flexbox layout.

Considerations

We're currently using LESS, though #9192 introduces SCSS. If we don't want to use 2 linting tools, we should migrate the LESS to SCSS and then use an SCSS linter.

[cjcenizal]

Closing this so we can focus on EUI. Our UIs should largely depend on EUI components anyway, so this problem may not be prevalent enough to guard against.