feat: allow simple filenames for synthesis

[roedoejet]

EveryVoiceTTS/EveryVoice#853

[joanise]

This \\ at the end yields something that is not cut-and-pastable as is. But if you use \\ \\ instead, the output has a single \ and then multi-line cut and paste works.

[joanise]

Another option that works even better is

    ```
    everyvoice synthesize text-to-wav logs_and_checkpoints/.../stage-2-last.ckpt \\
        --reference path/to/reference.wav  \\
        --text "Hello world" --text "How are you?"
    ```

this gives you something displayed as a code block, preserving indentation (the current solution does not), which is even better.

[joanise]

I suppose, though, cut and pasting in this case is a silly concept, maybe never mind. But I might still consider using the code block option for our examples.

[roedoejet]

yea, I don't think copy/paste is that important here. the code block option formats things a bit weirdly in my terminal, I think I'll leave this as is if that's alright

[joanise]

Looks good, thanks!

[joanise]

The current best practice recommendations are to add

   with:
     persist-credentials: false

every time we use actions/checkout, except for the rare cases where true is required (e.g., a workflow to needs to push to gh-pages), but I see that I have not started making that change anywhere in EV, nor even in Studio. It seems I only did it in g2p so far, so this comment is out-of-scope for this PR, but I wanted to mention it as something that I'll eventually spread everywhere, and is best practice to use when you create a new workflow.

[joanise]

Oh, another change that's going to be out of scope for this PR: this should be

        raise typer.BadParameter(
            "Got arguments for both --text and --filelist."
            " You can only synthesize using one of these options"
        )

instead of print and exit.

But I see that we have 39 instances of raise typer.BadParameter(...) and 43 instances of sys.exit(1) in the repo, so I'll make this a separate issue to do systematically: EveryVoiceTTS/EveryVoice#855