Desktop: Custom CSS: Add cm-listItem class to lines with list items, don't add region start/end markers for items that are always single-line

[personalizedrefrigerator]

Summary

The goal of this pull request is to simplify custom styling of the CodeMirror 6-based editor:

• With the legacy CodeMirror 5 editor, it was possible to style list items. With CodeMirror 6, this is only possible by referring to internal class names.
• The CodeMirror 6-based editor was adding unnecessary cm-regionFirstLine and cm-regionLastLine classes to regions that are always single-line.
  • This could complicate styling if headers or other affected items were present in a multi-line region (e.g. a blockquote).

Note

This currently targets release-3.1. Previously, certain list-specific custom styles were impossible (or very difficult) to migrate from CodeMirror 5 to CodeMirror 6.

Screenshot

CSS comparison

Before	After
Header 1	class="cm-h1 cm-headerLine cm-header cm-regionFirstLine cm-regionLastLine cm-line"	class="cm-h1 cm-headerLine cm-header cm-line"
Block math	First line: class="cm-mathBlock cm-regionFirstLine cm-line" Middle lines: class="cm-mathBlock cm-line" Last line: class="cm-mathBlock cm-regionLastLine cm-line"	First line: class="cm-mathBlock cm-regionFirstLine cm-line" Middle lines: class="cm-mathBlock cm-line" Last line: class="cm-mathBlock cm-regionLastLine cm-line"
Single-level ordered list	First line: class="cm-line" Middle lines: class="cm-line" Last line: class="cm-line"	First line: class="cm-orderedList cm-regionFirstLine cm-listItem cm-line" Middle lines: class="cm-orderedList cm-listItem cm-line" Last line: class="cm-orderedList cm-regionLastLine cm-listItem cm-line"
Single-level unordered list (bullet list/task list)	First line: class="cm-line" Middle lines: class="cm-line" Last line: class="cm-line"	First line: class="cm-unorderedList cm-regionFirstLine cm-listItem cm-line" Middle lines: class="cm-unorderedList cm-listItem cm-line" Last line: class="cm-unorderedList cm-regionLastLine cm-listItem cm-line"

Note: This pull request does not make it possible to consistently style even/odd list items differently (without a plugin).