This code hasn't been touched in a while, so it's probably good to bring
in the newest versions of the dependencies. We can easily tell if there
was any incompatible effect on the output.
The latest version of filenamify requires using ES modules. We also have
to adapt to a breaking change in regexpu-core (see
https://github.com/mathiasbynens/regexpu-core/pull/49).
Also convert the dependencies to devDependencies, since this tool is not
necessary for executing test262.
