Bump acorn from 6.1.1 to 6.4.1 (#273)

Bumps [acorn](https://github.com/acornjs/acorn) from 6.1.1 to 6.4.1.
- [Release notes](https://github.com/acornjs/acorn/releases)
- [Commits](acornjs/acorn@6.1.1...6.4.1)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Vincent Biret <vibiret@microsoft.com>
Co-authored-by: nikithauc <nikithauc@gmail.com>

Author: 3 people