feat(concatjs): allows the usage of @bazel/concatjs rules without Karma

[realtimetodie]

Seperates the @bazel/concatjs rules from the Karma web testing rules, by moving the karma_web_test rule and karma_web_test_suite rule into a dedicated subdirectory in packages/concatjs/web_test.

As a user, I want to use the @bazel/concatjs devserver rules without the requirement to install the io_bazel_rules_webtesting dependency. This dependency is only required by Karma web testing rules.

Before

load("@npm//@bazel/concatjs:index.bzl", "karma_web_test_suite")

After

load("@npm//@bazel/concatjs/web_test:index.bzl", "karma_web_test_suite")

The documentation was updated accordingly. The Karma web testing documentation in docs/Karma.md was updated. Previously, the documentation mentioned the @bazel/karma package.

[alexeagle]

It sounds good to reduce the accidental coupling here. However it's a breaking change to the public API so it has to wait for the next major release, following semver.