plugins: bump vimfiles/pack/plugins/start/vimtex (#151)

Bumps [vimfiles/pack/plugins/start/vimtex](https://github.com/lervag/vimtex) from `ded1d89` to `251615c`.
- [Release notes](https://github.com/lervag/vimtex/releases)
- [Commits](lervag/vimtex@ded1d89...251615c)

---
updated-dependencies:
- dependency-name: vimfiles/pack/plugins/start/vimtex
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>