Update plugins to support standalone components

- ThemedComponent wrappers should always import their base component. This ensures that it's always enough to only import the wrapper when we use it.
- This implies that all themeable components must be standalone

→ added rules to enforce this
→ updated usage rule to improve declaration/import handling

lint/test/testing.ts

@@ -20,6 +20,7 @@ import {
 themeableComponents.initialize(FIXTURE);
 
 TypeScriptRuleTester.itOnly = fit;
+TypeScriptRuleTester.itSkip = xit;
 
 export const tsRuleTester = new TypeScriptRuleTester({
   parser: '@typescript-eslint/parser',