Draft: Native CodeMirror 6 support

[personalizedrefrigerator]

Summary

At present, the Math Mode plugin doesn't work well in the CodeMirror 6-based beta editor. This pull request refactors the CodeMirror plugin to:

1. Use CodeMirror 6 APIs when running in CodeMirror 6
2. Use CodeMirror 5 APIs when running in CodeMirror 5.

Previously, CodeMirror 6 support was provided by Joplin's compatibility layer.

Why avoid the compatibility layer?

• Bugs
  • Currently, only the last line of math seems to be rendered correctly.
  • Issues with selection and inline results.
• The compatibility layer may be removed in the future.

Notes and resources

• Tutorial: Creating a Joplin plugin compatible with both CodeMirror 5 and CodeMirror 6 (takes a slightly different approach)
• CodeMirror 6 API documentation
• To allow correctly importing CodeMirror 6 libraries, the plugin build script was updated to the latest version.
• Many of the lines marked as added by git were moved from src/mathMode.ts, which was split into separate files.

To-do

• [ ] Improve refresh logic — currently, math isn't refreshed in some cases:
  • [ ] Adding a configuration line to a math block.
  • [ ] Converting a math block to a different type of code block.
• [ ] Remove changes to package-lock.json from this pull request — such changes are often difficult to review and can be made separately.
• [ ] Refactoring, code cleanup, and documentation
  • Consider taking the approach outlined in the [CodeMirror 5 compatibility documentation]((https://joplinapp.org/help/api/tutorials/cm6_plugin#codemirror-5-compatibility), where the CodeMirror 5 and CodeMirror 6 content scripts are bundled separately by Webpack. At present, CodeMirror 6
• [ ] More manual testing.