fix(bazel): ensure FTXUI is publicly accessible and add external smoke test (#1157)

- Set `visibility = ["//visibility:public"]` on the top-level `:ftxui` alias
  so the library can be consumed from external Bazel workspaces.
- Add `bazel_integration/` minimal external workspace to validate
  external usage via Bzlmod.
- Introduce `smoke` target that depends on `@ftxui//:ftxui`.
- Add CI job to build the smoke target using:
    --enable_bzlmod
    --override_module=ftxui=..
  This prevents regressions in visibility or public API changes.

Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

.bazelrc

@@ -1,4 +1,5 @@
 common --enable_bzlmod
+common --enable_workspace
 
 build --features=layering_check
 build --enable_bzlmod
@@ -6,4 +7,4 @@ build --enable_bzlmod
 build --enable_platform_specific_config
 build:linux --cxxopt=-std=c++20
 build:macos --cxxopt=-std=c++20
-build:windows --cxxopt=-std:c++20
+build:windows --cxxopt=/std:c++20