Implement feature supporting inline/block math

[strogonoff]

No description provided.

[strogonoff]

We can start with a non-inline block first, if it’s easier, but ideally it should support inline and non-inline math in the end.

[strogonoff]

For an example of a “feature”, see for example admonition blocks here: https://github.com/riboseinc/reprose/tree/master/src/features/admonition.

The schema part is responsible for contributing nodes and/or marks to overall ProseMirror schema used by the editor. That schema determines which blocks are supported, which blocks can go inside which blocks, and how to covert blocks to and from DOM/HTML.

The author part is responsible for contributing custom plugins that augment editor’s GUI (e.g., providing custom node views) and menu item configuration.

See typings in the files. Both files should export either a “feature”, or a function that takes some configuration and returns a feature.