Make sure default trigger name is unique

For func new, if users don't pass in a name, the default is trigger name, like "HttpTrigger". If a trigger with this name already existed, users would be asked about overwriting. But if the intention wasn't to overwrite (because the user didn't know about the default name), they'd have to cancel and start again. It could get annoying if they forget that the default name is the same each time and need to cancel and restart.

VSCode currently increments trigger name automatically to ensure there's no duplicate.