Read `.browserslistrc` file from Rust binary when no targets

[toastal]

Skimming the code, I didn't see any files being read for this setting. The documentation talks about leaving a browserslist in the package.json but the Rust binary doesn't require Node and by proxy a package.json. It seems Parcel v2 uses these RC files. Also not opposed to manually passing to the config file (in lieu of assuming $PWD/.browserslistrc)--it would just be more convenient to keep the settings in a separate file (which could also just be an lightningcssrc.{dhall,json,toml,yml} file).