modify or replace OverType

We are currently using the [OverType](https://overtype.dev/) library for syntax highlighting. Our biggest problem with it is that we are using it to *modify* text areas, but it's really built to *create* them. We're working against its nature in lots of places.

For example, right now we don't / can't enhance text areas by default, we only enhance very specific ones that we have targeted. This is because when we enhance a text area, we clobber its styling, and we have to manually put it all back how it was:

https://github.com/diffplug/gitcasso/blob/57fe5689b094d7ea9475e613fd033f4987c37fb0/src/lib/enhancers/github/GitHubPrAppendEnhancer.tsx#L71-L75

Also, whatever method it uses to listen for changes doesn't always work. For example, when images get dragged on to the GitHub textarea, the preview unsyncs briefly, but this can fix it:

https://github.com/diffplug/gitcasso/blob/57fe5689b094d7ea9475e613fd033f4987c37fb0/src/lib/enhancers/github/GitHubPrAppendEnhancer.tsx#L77-L82

Changes we have made:

- several removals of functionality related to
- added support for GitHub hashlinks e.g. `#34` https://github.com/diffplug/overtype/commit/f5149dce2125e1f3749961eaa4f12b75af648946