build: decouple JS API client bundling into an optional module

[dsmiley]

(AI gen, human reviewed)
The OpenAPI-generated JS client (npm install + babel build + browserify bundle) was embedded directly in :solr:webapp's build.gradle, making that module quietly depend on npm/node for something unrelated to compiling/packaging the webapp itself. Pull it out into its own module, :solr:webapp:js-client, which exposes the built bundle via a jsClientBundle configuration that :solr:webapp now consumes as a normal cross-project dependency (mirroring how it already consumes :solr:server's serverLib/solrCore and :solr:api's jsClient outputs), instead of :solr:webapp defining the npm tasks and reaching into its own local task outputs.

The new module is written in Kotlin DSL (build.gradle.kts) rather than Groovy, because Kotlin DSL's static typing advantages. It avoided a bug or two!

Also make the JS client bundle optional, mirroring the existing disableUiModule pattern:

• settings.gradle: new -PdisableJsClient=true flag (gradle.ext.withJsClient), excluding :solr:webapp:js-client from the build entirely when set.
• solr/webapp/build.gradle: the generatedJSClientBundle dependency is only added when the flag is on; the war{} block's from(configurations.generatedJSClientBundle) is left unconditional since an empty configuration is already a safe no-op.
• build.gradle: mirrors gradle.ext.withJsClient into rootProject.ext, matching the existing withUiModule convention.

Refactor gradle/node.gradle so it no longer hardcodes which projects need node/npm. Previously it named :solr:solr-ref-guide and :solr:webapp explicitly; now those projects apply the node-gradle plugin themselves in their own build.gradle(.kts), and gradle/node.gradle just reacts via allprojects { pluginManager.withPlugin(...) { ... } } to configure whichever projects end up applying it (proxy/registry override, pinned node version, .gradle/node workdir layout). This makes "does this module need node?" answerable by reading the module's own build file instead of a central list, and removes the maintenance burden of keeping that list in sync.

Verified end-to-end: default build (:solr:webapp:war) produces an identical war to before (same libs/solr/{README.md,docs/,index.js} contents); with -PdisableJsClient=true the war omits libs/solr entirely and zero npm/node tasks run; :solr:solr-ref-guide still exposes its node/npm tasks after self-applying the plugin.