feat: Add base for settings, and relocate venv-name default to it.

[DanCardin]

Relocates the .venv default venv name into a settings file that can be customized.

I couple of decisions I'm certain will end up getting questioned:

• etcetera vs dirs: Using etcetera preempts the moment where someone uses dirs to implement settings and annoy all the macos users who dont want their settings to live in Application Support/. (Like all the other rust tools using dirs/dirs-next that end up getting issues reported against them).
• Addition of toml_edit over toml. Mostly I'm just unfamiliar with toml and already have working code using toml-edit 😬 . Secondarily I figured long-term you'd probably implement uv init or uv add that will want to write toml, and you'll end up swapping to toml_edit then anyways.

---

If you're not ready to commit to specific settings, or otherwise already have a system (i.e. something from ruff) in mind that you'd want to replicate, feel free to close this. Rather than submitting an issue, I figured I could quickly do the thing I'd otherwise be requesting.

I didn't attempt to implement any tests in case this was quickly dead in the water.

[zanieb]

Hey Dan! Thanks for taking the time to open a pull request and sketch this out.

We're definitely not ready to commit to a configuration file yet, we want to approach it holistically since we'll need to remain backwards-compatible with what we choose for the foreseeable future. We might need to create an issue to discuss what people want from their configuration.

[zanieb]

I'm going to close this in favor of #1511 — please chime in there!