Option to build without `purs-backend-es` during development

[jacobpake]

Our Spago config specifies the purs-backend-es backend which produces better optimised builds than the standard PureScript backend. The standard backend is incremental but purs-backend-es is not - every change can take around ~10s or more to build, which is not bad, but may be unnecessary during development.

It would be nice to either remove this backend from the base config (and update scripts if necessary), or provide an alternative config without it.

For example, I am using a spago.dev.dhall with contents:

let base = ./spago.dhall

in { name = ""
   , dependencies = base.dependencies
   , packages = base.packages
   , sources = base.baseSources # [ "config/dev/**/*.purs" ]
   }

i.e. no backend specified.

During development I run spago -x spago.dev.dhall bundle-app -m Fluid -t dist/fluid/shared/fluid.mjs -p node -w -l which rebuilds on file changes and updates the Fluid executable. For small changes the whole build and bundle is less than a second.