Use browserslist entry in package.json with lightningcss-cli

[makkabi]

When using the lightningcss-cli, the only way to provide a browserslist target seems to be the --target option, while the browserslist entry in package.json (and also a .browserslistrc file) is beeing ignored. This means having to duplicate the browserslist target, but it would be much nicer if the lightningcss-cli would use the single definition in package.json like many other tools. The --target option could override the entry in package.json.

[toastal]

Looks like this could be a duplicate of #281

[LeoniePhiline]

I'll see if I can fix that.

[toastal]

@LeoniePhiline if you do, could you look at #281 too ($PWD/.browserslistrc)? If someone is using the Rust binary without npm, it would be weird to include a package.json just for browserslist.