Improve the way React component can extend Vanilla %placeholders

[bartaz]

React components should not include duplicates of Vanilla styling, especially Vanilla base, as it may cause conflicts when base styles override existing component styles (bug such as canonical/react-components#1041).

To avoid this we need to allow including SCSS placeholders without including any actual Vanilla styling.
This is possible with base placeholders via @include vf-b-placeholders, but is not possible with other more specific placeholders, such as included in vf-b-forms or other components.

We should extract the placeholders outside of regular Vanilla styles, so they can be included in React components without including the base/component styles themselves.

So for example any placeholders needed by forms, could be separated into vf-b-forms-placeholders (or be included in vf-b-placeholders).

Current workaround requires any Vanilla includes that are not just placeholders to be scoped within a component class name, as

.my-component {
  @include vf-b-forms; // Vanilla form styles scoped within the component
}