new command to set default editor opener type

[isabelizimm]

as part of #2933

to test with quarto extension SETUP:

1. launch positron dev
2. clone/open https://github.com/quarto-dev/quarto as workspace
3. check out branch save-editor-type Automatically open QMD files in visual/source editor if specified quarto-dev/quarto#577
4. run yarn dev-vscode
5. from debug tab, click play button (will launch another positron instance)
6. go to settings -> search "quarto editor" and swap between visual/source and open qmds. they should open with default editor.

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testing the changes:
7. in a qmd file, add editor: source to top yaml. check that it opens in source, regardless of default editor.
8. step 7 again, but with editor: visual
9. create a _quarto.yml file in the root of the workspace. add editor: source and see that qmds with no specific editor noted will open in source mode
10. step 9 again, but with editor: visual

[isabelizimm]

dont need this, can make all the changes in quarto!