Deprecate pulp browserify

[hdgarrood]

At the moment, the difference between pulp build and pulp browserify is a little confusing, and I think complicates both the CLI and the actual implementation unnecessarily. Furthermore, I'm not sure that it makes sense for Pulp to consider whichever JS tools it's using for bundling part of its public API (whether that be browserify, parcel, rollup, webpack, whatever). Instead, I'd prefer that we deprecate pulp browserify, and replace it with a flag to pulp build which specifies whether Pulp should resolve any require calls while bundling. Ideally, we could do this in such a way that we'd be free to switch between different JS tools without downstream users having to care.

[hdgarrood]

Note also that at some stage, purs bundle is likely to be simplified so that it no longer does dead code elimination, or even becomes deprecated; this is part of why I'd like to do this as well.