Allow inline TeX

[SeanDS]

It's possible to register a new format for the paragraph block. Add one to allow inline TeX.

[SeanDS]

This is rather difficult. The format API expects to transform text using an HTML tag and optional class, but still let the underlying text be editable. Formatting TeX code into mathematical markup requires replacing the user's text (the TeX code) with new elements, i.e. the rendered mathematics, and thus should not be editable. It may be possible to stop the elements from being editable with the contenteditable attribute but this might interfere with Gutenberg behaviour and would need to be undone when the user wants to edit the underlying markup.

I made an issue about this.