docs: add defaultBrowser config option

docs/app/references/configuration.mdx

@@ -146,6 +146,7 @@ For more options regarding screenshots, view the
 
 | Option                  | Default | Description                                                                                                                                                                                                                                                                                                                                           |
 | ----------------------- | ------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| `defaultBrowser`        | `null`  | The default browser to launch if the "--browser" command line option is not provided. This option only affects the first browser launched; changing this option after the browser is already launched will have no effect.                                                                                                                            |
 | `chromeWebSecurity`     | `true`  | Whether to enable Chromium-based browser's Web Security for same-origin policy and insecure mixed content.                                                                                                                                                                                                                                            |
 | `blockHosts`            | `null`  | A String or Array of hosts that you wish to block traffic for. [Please read the notes for examples on using this.](#blockHosts)                                                                                                                                                                                                                       |
 | `modifyObstructiveCode` | `true`  | Whether Cypress will search for and replace obstructive JS code in `.js` or `.html` files. [Please read the notes for more information on this setting.](#modifyObstructiveCode)                                                                                                                                                                      |