title: vscode api trigger on file selected in editor
body: <p>similar to
  <a href="https://stackoverflow.com/questions/60180833/get-currently-focus-file-via-vscode-extension-api">get currently focus file via VSCode extension API?</a></p>
  <p>when a user has openend (active file to edit, in the editor), the extension we are working on should update the multiroot workspace file. (in the path of the file is an identifier needed for another extension setting). This should be done only when the file is active and has a specific extension.</p>
  <p>Maybe with... registerTextEditorCommand  ?</p>
  <p>Thank you!</p>