build: emit and publish JS source maps for Sentry symbolication

[dcalhoun]

What?

Generate JavaScript source maps during the production build and publish them to
S3 so host apps can upload them to Sentry.

This is the GutenbergKit-side (Phase A) of a cross-repo effort. It produces
and delivers the maps; a follow-up in WordPress-iOS will fetch and upload
them to Sentry during its own release build. Symbolication doesn't take effect
until that host-side lane lands.

Why?

GutenbergKit editor exceptions currently appear in Sentry with minified,
unreadable frames (e.g. editor-CZ39QhNu.js:1:326003, see
JETPACK-IOS-1KAQ), making them
hard to triage. GutenbergKit produces the JS, but the host apps publish the
exceptions to Sentry — so GutenbergKit needs to emit source maps and hand them
to the host.

The approach mirrors the existing, proven gutenberg-mobile → host pipeline:
classic release/dist matching, with the host app performing the upload during
its own signed build. No Sentry SDK changes and no debug IDs.

How?

• vite.config.js: enable build.sourcemap: 'hidden' — emits .map files
  without a //# sourceMappingURL= comment in the shipped JS (the maps are
  stripped from the bundles, so such a comment would dangle). Dev-server
  debugging is unaffected; build.sourcemap applies only to production builds.
• Makefile: strip *.map from the iOS/Android native bundles in
  copy-dist-ios/copy-dist-android, so maps never ship inside the app (they'd
  bloat the binary and expose de-minified source) while dist/ keeps them for
  delivery.
• .buildkite/pipeline.yml: in build-react, package the maps into
  GutenbergKitSourceMaps.zip (preserving the assets/ layout), then strip
  maps from dist/ before dist.tar.gz — keeping every downstream consumer
  (including the Android publish, which copies dist/ directly) map-free.
• fastlane/Fastfile + .buildkite/release.sh: upload the zip to
  gutenbergkit/<version>/GutenbergKitSourceMaps.zip from publish_to_s3,
  alongside the XCFramework. Maps ship as a separate S3 object because SwiftPM
  binary-target consumers don't surface sibling files to the host build.

Publishing is optional end to end: artifact downloads tolerate a missing zip
and publish_to_s3 skips the upload when it's absent.

Testing Instructions

1. make build REFRESH_JS_BUILD=1.
2. Confirm maps are emitted: find dist -name '*.map' lists .map files.
3. Confirm maps are stripped from the native bundles:
   find ios/Sources/GutenbergKitResources/Gutenberg android/Gutenberg/src/main/assets -name '*.map'
   returns nothing.
4. Confirm the shipped editor chunks carry no //# sourceMappingURL= comment
   (the only hits are the third-party wordpress-globals/vips-worker files).
5. On CI, confirm the build-react step uploads a GutenbergKitSourceMaps.zip
   artifact and that dist.tar.gz contains no .map files.

Accessibility Testing Instructions

N/A — build tooling only; no UI changes.

[wpmobilebot]

XCFramework Build

This PR's XCFramework is available for testing. Add the following to your Package.swift:

.package(url: "https://github.com/wordpress-mobile/GutenbergKit", branch: "pr-build/554")

_{Built from c2507ee}