[8.x] Added a method to the Macroable trait that removes all configured macros.

[mad-briller]

This pull request adds a static flushMacros method to the Macroable trait that removes all the configured macros.

The primary use of this method is in a unit testing environment. When running multiple test cases that add static macros in the same process, there can be side-effects between test cases caused by the use of static members.

When combined with either the --random-order flag to PHPUnit or parallelized testing, these side effects can cause intermittent failures, as tests may or may not pass depending on the order / process they are ran in and the result of the side effects.
Ideally each unit test case is ran in a sanitized environment to ensure the developer is testing exactly what they intended to test, and aren't relying on any aspects setup outside of the current test case.

I was unsure as to whether the framework should enforce the running of flushMacros on contained Macroable classes, or whether the developer should handle this manually so i haven't done that.
Also enforcing that macros are always flushed is a not fully backwards-compatible change, whereas adding a method to allow developers to handle it themselves is.

Thanks for your time.

[nunomaduro]

This pull request looks good, and useful, to me.

[mad-briller]

rebased the branch because i noticed i had my work email set in git and not my noreply github one, whoops!