Create a BUILD file for compatibility with bazel

With this change, it's much easier for bazel projects to depend on
Catch. They just need to add:
  - In the workspace:
  ```
http_archive(
    name = "com_github_catchorg_catch2",
    urls = ["https://github.com/catchorg/Catch2/archive/v2.12.1.tar.gz"],
    strip_prefix = "Catch2-2.12.1",
    sha256 = "e5635c082282ea518a8dd7ee89796c8026af8ea9068cd7402fb1615deacd91c3",
)
```
  Or the appropriate version/sha256.
  - For the tests, assuming that `test_main.cc` contains the
  `CATCH_CONFIG_MAIN`:
  ```
cc_library(
    name = "test_main",
    srcs = ["test_main.cc"],
    deps = ["@com_github_catchorg_catch2//:catch2"],
)
```

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