extract_strings treats custom Draftail inline styles as blocks

[tognee]

Currently the list of inline tags is hardcoded:
https://github.com/wagtail/wagtail-localize/blob/main/wagtail_localize/strings.py#L9

This won't work when adding InlineStyleFeature using the register_rich_text_features wagtail hook.
Adding for example a generic inline style with a span will break it up as different string as if it was a block tag.

The best solution would be to retrieve the features from the richtext instance and follow all the InlineStyleFeature tags and attributes defined.

[zerolab]

@tognee it makes sense. I would make it so we the list is additive. That is, we start with our set list, and add anything in InlineStyleFeature

If you have the time, please do submit a PR. Will happily review