No longer resolves against 'browser' field

[tmcw]

Noticed that including simple-statistics is broken with d3-require 1.0.1 - this appears to be a result of simple-statistics relying on the browser field. I'll shortly add an unpkg field to simple-statistics to nail this down, but nothing that this is technically a regression from unpkg's (admittedly undocumented) behavior: it redirects

https://unpkg.com/simple-statistics -> https://unpkg.com/simple-statistics@6.0.0/dist/simple-statistics.min.js

whereas d3-require currently doesn't use unpkg's auto-resolution of a main file, and instead loads

https://unpkg.com/simple-statistics@6.0.0/dist/simple-statistics.js

From the main field in package.json, which is not browser-compatible.