Determine how bundling works with PureScript v0.15.0 version

[JordanMartinez]

See #401 (review) for more context.

I see a couple of ways we could still support bundling when using v0.15.0:

1. We don't bundle and simply fail when purs bundle is called
2. We don't bundle and fail when purs bundle is called. However, we recommend the user use pulp's in-built --then flag (e.g. pulp build --then "esbuild bundle --minify").
3. purs bundle implicitly calls esbulid with some basic arguments and allows additional arguments to be passed to it via an --esbuild-args flag or something.

If #401 gets merged in its present state, we'll default to 1. A future PR could update pulp to 2 or 3.

[thomashoneyman]

I don’t fully recall what Spago is doing, but I think we should follow their lead. If that’s too much work (Pulp is in maintenance mode, after all), then we should remove the command and throw an error recommending esbuild (or perhaps pointing at the PureScript migration guide)

[JordanMartinez]

Spago is going to use esbuild to bundle things.