Make library compatible with `symfony/phpunit-bridge`

[rdohms]

As reported in #11 originally by @drzraf

My symfony-based project depends on symfony/phpunit-bridge which download & install phpunit dynamically at runtime.
I'd rather prefer not to pull phpunit/phpunit when requiring phpunit-arraysubset-asserts.

[rdohms]

Regarding this, I believe phpunit-bridge is a complicated package with an anti-pattern the conflicts with distributing plugins for PHPunit.

Either the bridge should state that it provides PHPUnit or it should build its own scaffolding to be able to give you a way to require these packages. Their philosophy of a tool not affecting the project's dependencies means we need a different way to install these which is not composer. It's not the aim of this project to fix that.

Some of the discussion in this issue is relevant: symfony/symfony#27289
Since that has not been resolved, I feel it's misleading for this project to drop a dependency on something we really do depend on being present.

Marking this as won't fix until more clarity comes from the Symfony side.