feat: add ecs & phpstan code analysis

[khalwat]

Add ecs & phpstan code analysis

I also ran both ecs and phpstan, which cleaned up the code, and I manually fixed one error phpstan pointed out.

You can manually run either ecs or phpstan in local dev, and they will also automatically be run:

• Whenever you push to the v3-dev branch
• Whenever someone else does a PR

When someone does a PR, it'll look like this for them:

...and then once it has passed (or failed) it'll look like this:

[khalwat]

I did this only for the Craft 5 version of the plugin. If you want to backport it to Craft 4, it's the exact same everything for Craft 4.