Bazel: general improvements. (#1043)

* Bazel: general improvements.

Improve the Bazel build. Attempt to fix previous errors recorded while
trying to publish ftxui in the Bazel Central Registry:
- https://github.com/bazelbuild/bazel-central-registry/pull/4485
- https://buildkite.com/bazel/bcr-presubmit/builds/13601#01968b61-f5b2-4d16-94d0-c87a03a1a23b

Test against "recent" platforms
-------------------------------

Previously, I got the error:
```
gcc: error: unrecognized command line option '-std-c++20'; did you mean '-std-c++2a'?
```
This was due to using old distribution like ubuntu 2004. Test against
newer platforms only to avoid GCC version<-9.x.y

Downgrade gtest version.
------------------------

I suspect this caused the Bazel Central Registry error:
```
file:///workdir/modules/googletest/1.15.2/MODULE.bazel:68:20: name 'use_repo_rule' is not defined
```
Specifying using bazelmod fixes the issue. Thanks @robinlinden

Tag gtest as dev_dependency
---------------------------

Presumably, this should avoid dependants to fetch it?

Enable --features-layering_check
--------------------------------

Aka clang `-Wprivate-header`. Fix the encountered errors.

Use clang in the CI
-------------------

The CI was defining clang/gcc in the matrix, but was not using it. Fix
the bug.

tools/test_bazel.sh

@@ -0,0 +1,18 @@
+# This script tests the project with different versions of Bazel and compilers
+# locally. This avoids waiting on the CI to run the tests.
+
+for ver in \
+  "6.0.0" \
+  "7.0.0" \
+  "8.0.0"
+do
+  for cc in \
+    "gcc" \
+    "clang"
+    do
+      echo "=== Testing with Bazel ${ver} with ${cc} ==="
+      USE_BAZEL_VERSION=${ver} CC=${cc} bazel clean --expunge
+      USE_BAZEL_VERSION=${ver} CC=${cc} bazel build //...
+      USE_BAZEL_VERSION=${ver} CC=${cc} bazel test //...
+    done
+  done