Allow named args after unpack

Currently, argument unpacking and named arguments cannot be mixed
at all. This relaxes the restriction to allow
foo(...$args, named: $arg). The variant

Allow named args after unpack

Currently, argument unpacking and named arguments cannot be mixed
at all. This relaxes the restriction to allow
foo(...$args, named: $arg). The variant foo(named: $arg, ...$args)
is still forbidden, because we can't ensure that positional
parameters come before named parameters in that case (without more
intrusive changes). Effectively this just enforces a required style,
as the order of unpack and named args doesn't matter for the cases
where both could be well-defined.

ML discussion: https://externals.io/message/114589

Closes GH-7009.

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Implement named parameters

From an engine perspective, named parameters mainly add three
concepts:

* The SEND_* opcodes now accept a CONST op2, which is the
argument nam

Implement named parameters

From an engine perspective, named parameters mainly add three
concepts:

* The SEND_* opcodes now accept a CONST op2, which is the
argument name. For now, it is looked up by linear scan and
runtime cached.
* This may leave UNDEF arguments on the stack. To avoid having
to deal with them in other places, a CHECK_UNDEF_ARGS opcode
is used to either replace them with defaults, or error.
* For variadic functions, EX(extra_named_params) are collected
and need to be freed based on ZEND_CALL_HAS_EXTRA_NAMED_PARAMS.

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

Closes GH-5357.

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