Add ES-browser build

[mario-aleo]

Problem

Every build option that we have has the process validation, so if you try to import tippy.js to a project that is built for modern browsers and so it doesn't have a building step, it's going to fail because process is not defined at validation.ts:46.

Solution

It would be nice to have a .mjs output that does not transpile ES2015+ code and removes all references to process by assuming it is in a production environment.
This approach has been done at Redux reduxjs/redux#3143 and, as stated there by Tim, is similar to the one of Vue vuejs/vue#7110.

[atomiks]

Are you able to create a PR?

Also, can't you polyfill process in the browser? This would allow you to retain the warning messages which are helpful when using the lib, although the file size remains large with the warnings I suppose. You could also use the UMD version instead.

[IanVS]

This would be great. The ES build could also use import.meta.env instead of process.env to be compatible with modern build tools like astro, snowpack, and vite, and I believe webpack 5 too.