victory-vendor: script cleanup and exports enhancements

[ryan-roemer]

Adds the following to our package.json:exports for more flexible consumption:

• Add package.json:exports for package.json. This is generally useful to have for npm packages using exports to introspect the project itself.
• Add package.json:exports for lib/* consumers can point to CommonJS directly.
• Add package.json:exports for es/* consumers can point to ESM directly.

Additional things:

• Fix our build.js script to not errantly output an empty victory-vendor/d3-* directory.

Fixes #2250

[scottrippey]

Thanks for fixing that d3-* folder! I was pretty confused about its existence :)
Tested the builds locally, worked great.

[ryan-roemer]

@becca-bailey thanks for the review!

@scottrippey thanks for the review and comments! I think 537219d takes care of everything for you?

[scottrippey]

Looks great! Thanks for the answers.