fix: preserve per-build :js-options in merged-shadow-config

[ovistoica]

merged-shadow-config unconditionally replaces each build's :js-options with
{} for the root project or {:js-package-dirs [...]} for submodules. This
wipes out any per-build :js-options defined in shadow-cljs.edn.

This breaks projects that use :js-provider :external with :external-index,
because shadow-cljs never sees those options and falls back to the default
:shadow provider — silently skipping generation of the external index file.

Fix

• Root project: Don't touch :js-options at all (there's nothing to add)
• Submodules: merge into existing :js-options instead of replacing,
  so :js-package-dirs is added while preserving existing keys

Before

:js-options (if (= "" module-path)
              {}
              {:js-package-dirs [(str module-path "/node_modules")]})

After

(cond-> ...
  (not= "" module-path)
  (update :js-options merge {:js-package-dirs [(str module-path "/node_modules")]}))

I also updated the CHANGELOG with the fix. Please let me know if anything else is needed. Thank you for this great project!

[plexus]

tiny nitpick that this could have been (update :js-options assoc :js-package-dirs ...). But PR looks good, thanks!

[plexus]

Released in v0.48.215-alpha

[com.lambdaisland/launchpad "0.48.215-alpha"]                 ;; deps.edn
{com.lambdaisland/launchpad {:mvn/version "0.48.215-alpha"}}  ;; project.clj