feature request: allow matching multiple targets in key of target-defaults map

Currently target-defaults lets you reuse configurations for the same build target, e.g. :browser, but it does not let you reuse configurations for separate yet related build targets, e.g. :browser and :browser-test.

As a solution for this, would you consider a PR that allowed :target-defaults to match keywords in a set (or any collection)?

My use-case is sharing :js-options across related browser build targets:

 :target-defaults
 {:browser
  {:js-options
   {:extensions [".web.js" ".js" ".json"]
    :resolve {"react-native" {:target :npm
                     :require "react-native-web"}}}}
  :browser-test
  {:js-options
   {:extensions [".web.js" ".js" ".json"]
    :resolve {"react-native" {:target :npm
                              :require "react-native-web"}}}}}}

If :target-defaults matched build target configurations for sets, then the above could be:

 :target-defaults
 {#{:browser :browser-test}
  {:js-options
   {:extensions [".web.js" ".js" ".json"]
    :resolve {"react-native" {:target :npm
                     :require "react-native-web"}}}}

One consideration is ordering of merges if multiple :target-default are matched. Below ~8 items maps are an array-map, but still since Clojure maps don't guarantee order, this could give false assumptions if somehow someone's config unexpectedly got too big.

What do you think? Have you considered allowing multiple targets in a :target-default entry? Do you have other ideas for how to address shared configs for related build targets?

No implementation files or tests are named. Start by locating the target-defaults handling and its existing tests, then trace how target keys are matched and configurations are merged. Done means defining multiple-target matching and deterministic merge behavior, with tests covering shared targets and overlapping matches.