Check the purity declarations of methods without a body

[zonuexe]

Follow-up to #6018 (comment). PureMethodRule runs on MethodReturnStatementsNode, which only methods with a body get, so PHPStan never checked the purity declarations of abstract and interface methods:

interface Foo
{
	/** @phpstan-pure */
	public function pureByRef(int &$p): int; // not reported

	/**
	 * @param-out int $count
	 * @pure-unless-parameter-passed $count
	 */
	public function replace(string $subject, int &$count): string; // not reported
}

PHPStan reports the same declarations on a method with a body as pureMethod.parameterByRef and pureMethod.nonOptionalParameterPassed.

Changes

• FunctionPurityCheck::checkSignature() holds the checks that need only the signature: parameterByRef and void for @phpstan-pure, redundantUnlessCallable for @pure-unless-callable-is-impure, and nonOptionalParameterPassed / parameterPassedNotByRef for @pure-unless-parameter-passed. check() calls it first, so PureFunctionRule, PureMethodRule and PurePropertyHookRule report the same errors as before.
• The new PureAbstractMethodRule runs checkSignature() on InClassMethodNode for methods without a body, with the same pureMethod.* identifiers.
• The rule can report existing @phpstan-pure interface methods, so I registered it on level 2 (like PureMethodRule) behind the reportAbstractMethodPurity bleeding-edge toggle.