[Security] Bump acorn from 5.7.3 to 6.4.2

Bumps [acorn](https://github.com/acornjs/acorn) from 5.7.3 to 6.4.2. **This update includes security fixes.**
- [Release notes](https://github.com/acornjs/acorn/releases)
- [Commits](acornjs/acorn@5.7.3...6.4.2)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>