Get libicu from Bazel icu module

Bazel now has an ICU module (https://registry.bazel.build/modules/icu), so it now may be possible to use that hermetically instead of relying on a system library for it.

I've looked into this a little bit and it doesn't seem that Bazel's the icu module has a shared library target, which may be necessary for this to be usable here.

Start by reading the Bazel ICU module at https://registry.bazel.build/modules/icu and determine whether it exposes the shared library target this repository requires. Compare that capability with the current system-library dependency; done means ICU can be consumed hermetically through the module, or the missing blocker is documented clearly.

Written by the indexing model from the issue text.