Upgrade to CodeMirror6

[mb21]

• basic implementation with customized styling
• grey background for markdown code blocks and code spans (see https://discuss.codemirror.net/t/how-to-define-highlighting-styles-for-blocks/4029/4)
• check out history and search from https://github.com/codemirror/basic-setup/blob/main/src/codemirror.ts
• uncomment and fix code around on.addBold (see https://discuss.codemirror.net/t/keymap-for-bold-text-in-lang-markdown/3150)
• re-implement indent wrap (like we had for CodeMirror 5) so that e.g. line-wrapped text of a list item continues indented after the list marker see https://discuss.codemirror.net/t/making-codemirror-6-respect-indent-for-wrapped-lines/2881/8)
• uncomment and fix code for scrolling preview (see In split view, editor and preview views do not scroll in sync #95)
• check whether cursor always stays (instead of jumping to the beginning) when we update the editor from the state.doc.md, or whether that's not a problem

I'll see whether I can fix all these things. But pull requests for any or all of them welcome, just fork from this branch!