Split preview mode into separate feature flags (#14823)

I think this would give us better hygiene than a global flag. It makes
it easier for users to opt-in to overlapping features, such as Python
upgrades and Python bin installations and to disable warnings for
preview mode without opting in to a bunch of other features. In general,
I want to reduce the burden for putting something under preview.

The `--preview` and `--no-preview` flags are retained as global
overrides. A new `--preview-features` option is added which accepts
comma separated features or can be passed multiple times, e.g.,
`--preview-features add-bounds,pylock`. There's a `UV_PREVIEW_FEATURES`
environment variable for that option (I'm not sure if we should overload
`UV_PREVIEW`, but could be convinced).

Cargo.lock

@@ -5037,6 +5037,7 @@ name = "uv-configuration"
 version = "0.0.1"
 dependencies = [
  "anyhow",
+ "bitflags 2.9.1",
  "clap",
  "either",
  "fs-err 3.1.1",
@@ -5061,6 +5062,7 @@ dependencies = [
  "uv-pep508",
  "uv-platform-tags",
  "uv-static",
+ "uv-warnings",
 ]
 
 [[package]]