Warn about potential mistakes during development

[kripod]

Motivation

Similarly to how Emotion and Styled Components work, warnings should be emitted to the console during development.

Details

Cases to handle:

• Invalid value passed to a content property. (See: Emotion – This may be checked statically by linting, as proposed in #11.)
• Too much CSS is injected dynamically from the same place. (See: Styled Components – This might only be handled by view library bindings.)
• Non-atomic pseudo selector is being used outside selectors, targeting e.g. data attributes or nth elements.
• Advanced selectors not starting with "&". – (This may be checked statically by linting, as proposed in #11.)
• Inconsistent conditional rule order (e.g. media queries in mixed order instead of a mobile-first or desktop-first approach) – (This should probably be checked statically by linting.)