Migrate bazel build to bzlmod

[reutermj]

Migrate existing bazel build files to bzlmod. This allows flecs to be used as a bazel dependency using git_override.

bazel_dep(name = "flecs")
git_override(
    module_name = "flecs",
    remote = "https://github.com/SanderMertens/flecs.git",
    commit = "bf4b6b6302dd19eb11100c44d81da5b3065db846",
)

Existing WORKSPACE based bazel configuration is maintained if being built with older versions of bazel.

amalgamated source and header are moved to the distr folder to fix bazel build issue on windows.