New Courtesy Accidentals plugin

[XiaoMigros]

The current courtesy accidentals plugin is limited in where it adds courtesy accidentals, and the UI for configuring the settings isn't particularly good-looking. In an attempt to tackle both of those issues, here's a new Courtesy Accidentals plugin.

• [x] I signed the CLA
• [x] The title of the PR describes the problem it addresses
• [x] Each commit's message describes its purpose and effects, and references the issue it resolves
• [ ] If changes are extensive, there is a sequence of easily reviewable commits
• [x] The code in the PR follows the coding rules
• [x] There are no unnecessary changes
• [x] The code compiles and runs on my machine, preferably after each commit individually
• [ ] I created a unit test or vtest to verify the changes I made (if applicable)