Fix false positives for @import detection

[rmunch]

The detective was matching things like @media queries and @keyframes when looking for imports. This updates it to specifically match @imports. Thanks!

[rmunch]

@XhmikosR I ran into an additional bug, and realize the gonzales-pe package that is used to parse .less files appears not to have been updated since 2019. I wonder if you'd be interested if I open a pull request to update this package to use postcss and postcss-less instead? I also see other detectives like detective-sass are using gonzales-pe, so as an alternative, maybe it'd make sense to update detective-postcss to be able to accept plugins to handle each case? Thanks!

[rmunch]

For now, I've patched precinct for my project to be able to register custom detectives similarly to what's mentioned here, and am pointing to a custom script that uses postcss-less, though I'm happy to make a PR if it would be helpful. Thanks!