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.

Start with the `func new` command and trace how the default trigger name, such as "HttpTrigger", is chosen before the overwrite prompt. Compare the behavior with VSCode's automatic incrementing; done means an omitted name receives a unique trigger name without asking to overwrite an existing trigger unexpectedly.

Written by the indexing model from the issue text.