Detect changes to markdown syntax and open in code editing mode

Splitting out an idea laid out in https://github.com/nextcloud/text/issues/593#issuecomment-1323244299 as suggested by @PhrozenByte in https://github.com/nextcloud/text/issues/593#issuecomment-1324202910

Background

We want to enable the best possible editing experience to users while making sure that markdown syntax is preserved for those who care about it by default. There's a long discussion about this in #593.

A lot of work is going into preserving the markdown syntax already but the architecture of texts wysiwyg editing and limitations of the underlying libraries make it close to impossible to preserve all syntax out there.

Proposal

(updated to match the current state of discussion)

• When opening a .md file... check if it would be serialized to the same code.
• If serializing the document would change the syntax open the document in code editing mode.
• Have an info, why there's no wysiwyg functionality.

Possible Additions

• It would be good to allow switching to wysiwyg - but that would not be required for a start from my point of view.
• Make the syntax match the source as close as possible. If we manage to match the syntax we can have wysiwyg more often.

Alternatives

Start by reviewing the linked discussion in #593, then trace how .md files are opened and how documents are serialized. The intended result is to detect serialization changes and open affected documents in code editing mode, with an explanation that WYSIWYG editing is unavailable.