Add an `EuiThemeProviderDecorator` to kibanaReact which uses the Storybook globals to set the EUI theme.
Add global decorators to the APM and Observability plugins so all stories are wrapped in the `EuiThemeProviderDecorator`, and they don't need to specify it in the stories.
Add [jest setup helpers recommended by @storybook/testing-react](https://github.com/storybookjs/testing-react#global-config).