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>

Treat attribute argument lists like normal argument lists

Allow trailing comma. Syntactically allow unpacking, but forbid it
during compilation.

The trailing comma test-case is

Treat attribute argument lists like normal argument lists

Allow trailing comma. Syntactically allow unpacking, but forbid it
during compilation.

The trailing comma test-case is adopted from GH-5796.

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