Bump acorn from 7.1.0 to 7.3.1 in /forms-flow-forms (#21)

Bumps [acorn](https://github.com/acornjs/acorn) from 7.1.0 to 7.3.1.
- [Release notes](https://github.com/acornjs/acorn/releases)
- [Commits](acornjs/acorn@7.1.0...7.3.1)

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