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.

.bcr/presubmit.yml

@@ -1,21 +1,36 @@
 matrix:
-  platform:
-  - centos7
-  - debian10
-  - ubuntu2004
-  - macos
-  - windows
-  bazel: [6.x, 7.x, 8.x]
+  bazel:
+    - 7.x
+    - 8.x
+    - rolling
+  unix_platform:
+    - debian11
+    - ubuntu2204
+    - macos
+    - macos_arm64
+  win_platform:
+    - windows
+
 tasks:
-  verify_targets:
-    name: Build and test.
-    platform: ${{ platform }}
+
+  unix_test:
+    name: Verify build targets on Unix
+    platform: ${{ unix_platform }}
     bazel: ${{ bazel }}
+    build_flags:
+    - --cxxopt=-std=c++20
     build_targets:
-    - '@ftxui//:ftxui'
-    - '@ftxui//:screen'
     - '@ftxui//:dom'
     - '@ftxui//:component'
-    test_targets:
-    - '@ftxui//:tests'
+    - '@ftxui//:screen'
 
+  windows_test:
+    name: Verify build targets
+    platform: ${{ win_platform }}
+    bazel: ${{ bazel }}
+    build_flags:
+    - --cxxopt=/std:c++20
+    build_targets:
+    - '@ftxui//:dom'
+    - '@ftxui//:component'
+    - '@ftxui//:screen'