Use the package.json "browser" field to toggle Node.js-specific functionality

[goto-bus-stop]

Following from the discussion in
webpack/webpack#8826, this is how it'd work
with the existing "browser" field rather than any new special comment or flag.
Feel free to close this PR if you don't like it!

By using the "browser" field, we can tell bundlers to use the
infer-label.browser.ts file instead of infer-label.ts. This obviates
the need for the customRequire hack and lazy loading fs. In this case
it has the side benefit of omitting the Node-specific inferLabel
function from the bundle entirely, whereas previously it would still be
included, just never run.

An alternate solution is to use:

"browser": {
	"fs": false
}

Then the fs module will return an empty object when bundled;
inferLabel would need to do isNode && fs && fs.readFileSync to check
if files can be read.

AFAIK, the only bundler this doesn't work in at the moment is Meteor, I
don't know if that's a blocker for you.

[sindresorhus]

Alright. This seems like the best solution for now... Thanks :)