feat: add VP_VERSION env var for install script compatibility (#40)

## Summary

- Set both `VP_VERSION` (new) and `VITE_PLUS_VERSION` (legacy) env vars
when invoking the vite-plus install script
- Ensures the action works with vite-plus versions before and after the
`VP_*` env var rename (voidzero-dev/vite-plus#1166)
- Includes a TODO comment to remove `VITE_PLUS_VERSION` once old
versions are no longer supported

## Test plan

- [x] CI passes with current vite-plus release (reads
`VITE_PLUS_VERSION`)
- [x] After voidzero-dev/vite-plus#1166 merges, verify install still
works (reads `VP_VERSION`)

🤖 Generated with [Claude Code](https://claude.ai/claude-code)

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Signed-off-by: MK (fengmk2) <fengmk2@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: fengmk2