Add support for Project Browser

lostcarpark → credited cmlara → .

I've completed the addition of a "Tools->Project Browser->Clear storage" option.

Had some difficulty getting tests to run for Next Minor in GitLab CI, but thanks to @cmlara on Slack, I figured out the correct versions to specify in composer.json.

Still getting a PHPStan error for next minor.

PHPStan error is due to Project Browser updates for Package Manager being addressed in 📌 Conflict with Project Browser <2.1.0 and Automatic Updates <4.0.0 Active , so not a concern for this module.

I've added a test of pressing the "Clear storage" menu, and verifying the correct response.

I'm moving this to RTBC.

Merged and released version 1.2.0.