Improve behavior of code_actions_on_format migration

[probably-neb]

Closes #40334

Release Notes:

• Fixed an issue where auto-format would break after updating to v0.208.4 or v0.209.0-pre. If you were affected by this, you need to update any "formatter" sections in your settings.json file that only have code action formatting steps and add an "auto" step (or whatever you previously had configured under "formatter" at the top level of the settings)

For example:

{
    "formatter": "prettier",
    "languages": {
        "TypeScript": {
            "formatter": [
                { "code_action": "source.fixAll.eslint" }
            ]
        }
    }
}

Should become:

{
    "formatter": "prettier",
    "languages": {
        "TypeScript": {
            "formatter": [
                { "code_action": "source.fixAll.eslint" },
                "prettier"
            ]
        }
    }
}

Additionally, if you did not have a formatter configured at the top level of your settings, you can run the zed: open default settings action via the command palette to see what the default formatter configuration Zed uses for your specific language is.

If you have not modified your settings since hitting the backup and migrate settings button after updating to the most recent version, you may also restore the contents of your settings from the settings_backup.json file that can be found in the same directory as your settings.json file, then re-run the migration which will restore the behavior from before the update automatically.

[probably-neb]

Closed in favor of #40409