feat: support package exports and custom paths

[KaelWD]

Requires https://github.com/pastelsky/package-build-stats/pull/52

fixes #218 fixes #379

Next step would be to parse package.exports and generate a list of available entrypoints and their sizes, probably have to skip anything with wildcards though.

[kurtextrem]

@pastelsky any updates here?

[radist2s]

@KaelWD, thank you so much for putting together this PR. @pastelsky, this PR is quite important in my opinion, for example, my library doesn't display the bundle size even though it's done according to the standards. The reason for this is the absence of module and main in package.json, which is not a problem for modern Node.js (>12.x).