Add Bazel support

[Vertexwahn]

This PR helps users of the Bazel build system to integrate xtl in their builds.

Supporting two build systems (CMake and Bazel) is of course more effort to maintain but helps to spread xtl.
In the worst case the Bazel build does simply not work - in the best case, someone who uses Bazel can benefit from it.

Also, other projects support CMake and Bazel side by side (e.g. Catch2, gtest, etc.)

[JohanMabille]

Hi @Vertexwahn , and thank you for your contribution.

In the worst case the Bazel build does simply not work [...]

I am not fan of providing sometihng that potentially does not work, meaning this build should be tested and maintained. However, we have limited bandwidth, and I won't be able to maintain this myself; so is that something you plan to maintain on the long run?

Another approach could be to have the Bazel build of xtl in a dedicated repo.

[Vertexwahn]

@JohanMabille I can maintain it at least for the next 6 months. After this period we can decide if this is dropped again or if I can continue.

[Vertexwahn]

Moved it to Bazel Central Registry: https://registry.bazel.build/modules/xtl