Fix high dependency alerts in slide packages

[lancewillett]

Summary

• Updates underscore, mustache, and grunt in the three reveal.js slide package manifests.
• Keeps the change manifest-only because this repo does not currently include npm lockfiles for these packages.

Validation

• jq empty on all three changed package.json files
• npm install --package-lock-only --ignore-scripts --dry-run --audit=false --fund=false --engine-strict=false --legacy-peer-deps --prefix <slide package> for each changed package
• git diff --check

Note: npm reports the existing node: ~0.8.0 engine metadata as unsupported on the current local Node version. Dependency resolution still completed in dry-run mode.