Fix the error message of attribute flag validation

Change Attribute Syntax from @@ to #[]

Implement Shorter Attribute Syntax

RFC: https://wiki.php.net/rfc/shorter_attribute_syntax

Closes GH-5796.

Co-authored-by: Martin Schröder <m.schroeder2007@gmail.com>

Implement Attribute Amendments.

RFC: https://wiki.php.net/rfc/attribute_amendments

Support for attribute grouping is left out, because the short
attribute syntax RFC will likely

Implement Attribute Amendments.

RFC: https://wiki.php.net/rfc/attribute_amendments

Support for attribute grouping is left out, because the short
attribute syntax RFC will likely make it obsolete.

Closes GH-5751.

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