ci: improve Bazel build verification and update .bazelignore

- Created BUILD.bazel files for all subprojects to enable recursive build verification.
- Updated root BUILD.bazel to track all project source files.
- Cleared .bazelignore of project directories to allow Bazel to scan them.
- Excluded files with colons from Bazel globs as they are invalid labels.

lit-showcase/BUILD.bazel

@@ -0,0 +1,5 @@
+filegroup(
+    name = "srcs",
+    srcs = glob(["**"]),
+    visibility = ["//visibility:public"],
+)