Add additional methods for reflections that have attributes

Description

All currently attributable reflections have a single method relating to attributes.

getAttributes(?string $name = null, int $flags = 0): array

Additional methods for dealing with attributes would be a nice improvement and would simplify common approaches. I'm proposing the following methods.

getAttribute

Get a single instance of ReflectionAttribute, following the same rules as getAttributes, minus the fact that $name is required.

getAttribute(string $name, int $flags = 0): ?ReflectionAttribute

If more than one attribute matches, an exception should be thrown.

hasAttribute

Functionally identical to getAttribute, except it will return true if one or more matches are found, otherwise it returns false.

hasAttribute(string $name, int $flags = 0): bool

getNumberOfAttributes

Again, follows the same rules as getAttributes, but will return the number found. This particular method is based on the ReflectionFunctionAbstract::getNumberOfParameters method.

getNumberOfAttributes(?string $name = null, int $flags = 0): int

首先定位可附加属性的反射对象现有的 getAttributes 实现，以及相关的 ReflectionAttribute 测试。以 ReflectionFunctionAbstract::getNumberOfParameters 作为参考先例，然后针对所有可附加属性的反射类型，验证所提议方法的匹配、flags、重复项处理和返回值。