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

Start by locating the existing getAttributes implementations for attributable reflections and the related ReflectionAttribute tests. Use ReflectionFunctionAbstract::getNumberOfParameters as the referenced precedent, then verify matching, flags, duplicate handling, and return values for the proposed methods across all attributable reflection types.