docs: add defaultBrowser config option

[alexsch01]

For cypress-io/cypress#30517

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[MikeMcC399]

What about https://docs.cypress.io/app/references/module-api#Options ?

Should there be an entry there as well?

• (I may have been misled due to the comment in cypress-io/cypress#30517 (review))

  Approving the addition of defaultBrowser to the Module API as a code reviewer

[alexsch01]

Wouldn't it make more sense to put this in the https://docs.cypress.io/app/references/configuration#Browser section?

changed, I think that makes sense