Add @psalm-pure to the typeToString method

[luukvhoudt]

When extending the Assert class it can be difficult to keep custom methods pure, especially when opting for the use of the Assert::typeToString method. This results in the following Psalm error at the call to the typetoString method:

ImpureMethodCall: Cannot call an impure method from a pure context

Currently there are two options to avoid the error:

• Don't mark your custom method pure.
• Suppress the ImpureMethodCall psalm error for your custom pure method.

Ideally you neither pick one of those options.

This PR fixes the error for good error by marking the Assert::typeToString method as @psalm-pure.

[shadowhand]

@luukvhoudt please rebase so that CI will run.

[shadowhand]

Needs a rebase for CI fixes (see #304)

[shadowhand]

Thank you!