Add a Bazel-oriented source package profile and extract the release logic

[frankslin]

Adds OPENCC_SOURCE_PACKAGE_PROFILE=bazel, a trimmed source package that keeps the Node.js and Python binding sources so the //node and //python Bazel targets stay buildable and testable from the tarball, and moves the build/smoke-test logic out of release-source.yml into a reusable shell script.

Detailed Changes:

• CMakeLists.txt:
  • Split the trimmed ignore list into a shared part and a binding-sources part (node/, python/, scripts/), so the new "bazel" profile keeps them while "opencc" drops them. The "opencc" package contents are unchanged.
  • Both trimmed profiles now drop the npm/PyPI packaging entry points (package.json, package-lock.json, pyproject.toml, setup.py, MANIFEST.in, plugins/jieba/node/); Bazel does not need them.
  • Replace the "/python[^/]/" pattern, which also happened to exclude deps/marisa-/bindings/python[3]/, with explicit patterns so the OpenCC python/ directory can be kept independently.
• scripts/build-source-package.sh (new): build / smoke-test / all subcommands taking a profile. Builds the CPack archive, repacks it for reproducibility, then smoke-tests it with CMake and Bazel; the "bazel" profile additionally runs the full CI Bazel build and test target patterns. Explicit target patterns are used instead of //... because the vendored deps/googletest-1.15.0 BUILD files do not load under current Bazel versions. Fixes the CMake smoke-test target name, which was opencc-jieba (the library OUTPUT_NAME) instead of the actual target opencc_jieba.
• .github/workflows/release-source.yml: run the script once per profile via a build matrix; artifacts and release assets are uploaded per profile.
• Docs: update the source package section in AGENTS.md and the NEWS.md entry; ignore /build-src.