filter to add autosuggest styles while indexing

[hugosolar]

Description of the Change

The filter ep_autosuggest_styles_if_idexing should allow enqueueing autosuggest styles even if the website is being reindexed.
We have a project where we rely on those styles on the frontend but we're not relying on the ElasticPress built-in endpoint for autosuggest so not having those styles break the site appearance

Closes #2841

Alternate Designs

For now, we add a fix at the theme level where we're enqueuing the styles if are not enqueued but I think it could be helpful to force the styles if it's needed using a filter

Possible Drawbacks

since this change was introduced as a fix to a JS error, there are no possible side-effects since are just the styles

Verification Process

Fork the project and test my branch in my local environment. Ran a full site reindex and then test the filter is working properly

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Changelog Entry

Added - ep_autosuggest_styles_if_idexing filter to enqueue autosuggest styles even if index is running

Credits

Props @hugosolar