Add generic attr-non-space-content evaluate method

[WilcoFiers]

This generic evaluate method could look something like this:

const label = virtualNode.attr(options.attr);
return !!(label ? axe.commons.text.sanitize(label).trim() : '');

This will replace a number of non-empty-* evaluate methods.

[padmavemulapati]

devTask

[jeankaplansky]

no user-facing docs required.