Allow content: 'none' and 'initial' for pseudo-elements

[frontendj]

Rule 'content-property-no-static-value' doesn't allow us to have such constructions:

&::before { content: none; }

&::before { content: initial; }

Since they are widely used for decorative purposes, I'd suggest supporting them.
Also since the list of exceptions became bigger, maybe it makes sense to slightly optimise it.