Add support for importing existing notes

[ybbarc]

Allows importing existing markdown files as notes from:

• Folder: You can specify a notes folder in the addon settings. The import command will search for a markdown file with the same name as your game (excluding any special characters) and if found, will import it into PlayNotes
• File: If you want to specify a separate file, you can use the override.

I have verified that:

• These changes work, by building the extension and testing.
• That the changes comply with the rules indicated in the repository.
  TODO: Add localization
• Pull request is targeting master branch.