Fix pkg.browser mappings issue by specifying a value of false

[allex]

Fix pkg.browser with some maps specifying a value of false to ignore a module. (https://github.com/defunctzombie/package-browser-field-spec)

...
"browser": {
    "module-a": false,  // ---->>> map to empty.js
    "./server/only.js": "./shims/server-only.js"
}

[bterlson]

@guybedford @TrySound @lukastaegert @keithamus anything I can do to help with this PR? I'm trying to migrate some projects to rollup and being able to ignore node built-ins that I know for sure won't be called in the browser (ala webpack) would be really nice.

In the meantime, I'm rolling my own shims with rollup-plugin-shim, but it's frustrating because I have to shim each export which I think this PR obviates. Let me know if there's a better workaround!

[lukastaegert]

Will try to have a look tomorrow

[bterlson]

@lukastaegert yay! I'm here all day (PST), let me know if I can help!

[lukastaegert]

Looks good apart from the two remaining suggestions

[lukastaegert]

Released