js: move the Astro, MDX, Svelte and Vue tests and READMEs into families/js #9215

Part of #6508 (and #6054). Second slice of the js move. Do after the first slice (scaffold families/js = rules_dx_js, plus the JavaScript and TypeScript tests) has merged.

Where

• astro/tests, mdx/tests, svelte/tests, vue/tests and their README.md files (about 165 files; rules, ide/ and toolchains stay in core).
• Root BUILD.bazel: the # gazelle:exclude lines and exports_files / real_source_target README entries for these four dirs.
• Labels into these dirs from outside: quality/tests/BUILD.bazel:104,112 (//svelte/tests/consumer:app_consumer_test.rs), tools/testing/fixtures/consumer_maven/BUILD.bazel:7 (visibility), tools/docs/support_matrix/languages.bzl:322 (//mdx/tests/app:site_static_test).
• modules/families.json js entry (dirs) and core entry.

Fix

1. git mv each <lang>/tests and <lang>/README.md to families/js/<lang>/. Follow the jvm move (jvm: move the Java, Kotlin and Scala tests and READMEs into families/jvm (part of #6503) #8218) exactly.
2. Rewrite outside labels to @rules_dx_js//<lang>/tests/...; inside the family use //... for family labels and @rules_dx//... for core labels. Widen core visibility to public only where a family target needs it.
3. Move the gazelle excludes into families/js/BUILD.bazel and add the READMEs to its corpus_markdown.
4. Update modules/families.json (js dirs gains astro, mdx, svelte, vue; core keeps the root dirs) and docs/README.md links.
5. Tests: tools/dx test //modules:all //quality/tests/... //tools/docs/..., cd families/js && bazel test //..., then tools/dx test --affected.