Remove dependency on gulp-util

[jonpearse]

gulp-util was deprecated a while back, and while this isn’t the end of the world, it in turn unfortunately depends on a version of lodash with a critical vulnerability.
This was causing security alerts on a project I’m working on, and in turn was causing our CTO to jump up and down in an amusingly animated manner :)

Either way, I’ve switched out replaceExtension calls to use the replace-ext module, and gutil.File to use standard Vinyl objects. I also switched instances of new Buffer() to Buffer.from(string) to get rid of some deprecation warnings.
I’ve not bumped any other deps—I’ll leave that to your discretion. Tests seem to pass OK, but apologies in advance if I’ve missed anything else.

Otherwise, thanks for making this plugin—switching to it shaved a couple minutes off our project’s build time, which makes me all kinds of happy!