feat(create-vite): support Vite+ (vp) package manager

[fengmk2]

Description

When create-vite is run via vp dlx create-vite (Vite+), it currently falls back to npm commands because the underlying package manager (pnpm/npm/yarn/bun) overwrites npm_config_user_agent, hiding that vp launched it.

This teaches create-vite to recognize vp:

• Detect vp from a dedicated VP_USER_AGENT env var that vp sets (falls back to npm_config_user_agent as before).
• getRunCommand emits vp dev; getInstallCommand already yields vp install.
• getFullCustomCommand maps npm create to vp dlx create- and npm exec to vp dlx (same shape as the existing bun/pnpm/deno branches).

Result: a vp dlx create-vite run finishes with vp install / vp dev, and template custom commands use vp dlx.

Why vp dlx create- and not vp create

npm create X is defined as npm exec create-X, and vp dlx create-X is the exact structural equivalent (dlx = exec), forwarding all positionals and flags straight to the create-* binary. vp create was considered but is unsuitable here: it consumes the target-dir positional itself (uses its own --directory flag), so the TARGET_DIR convention is dropped, and the npm create -- vike@latest --vue TARGET_DIR templates need their template flags after a -- separator that a prefix-only replacement cannot produce. vp create is also a richer, opinionated flow (adds agent/editor/git/hooks setup) rather than a transparent passthrough.

Note

The companion change that makes vp set VP_USER_AGENT ships in the vite-plus repo (voidzero-dev/vite-plus#1958). Until that lands, behavior is unchanged for all existing package managers.

Test

Added a create-vite CLI test asserting that VP_USER_AGENT takes precedence over npm_config_user_agent and produces vp install / vp dev.

[fengmk2]

@codex review