Add @only-named-arguments annotation

[GromNaN]

Fixes #11763

Adds a @only-named-arguments docblock annotation that requires callers to use named arguments. This is useful for libraries that want to enforce named-argument call sites as part of their BC policy — allowing parameter reordering or removal without breaking positional callers.

When a positional argument is passed to a function or method annotated with @only-named-arguments, Psalm emits the new PositionalArgumentNotAllowed issue. Spread/unpack calls are also covered: foo(...[0, 1]) triggers the issue while foo(...['a' => 0, 'b' => 1]) is allowed.

Using both @only-named-arguments and @no-named-arguments on the same declaration is flagged as InvalidDocblock.

For variadic parameters, the inferred type of ...$args becomes array<string, T> (instead of list<T> with @no-named-arguments or array<array-key, T> without annotation).

@only-named-arguments takes priority over the global allowInternalNamedArgumentCalls="false" and allowNamedArgumentCalls="false" config options, allowing library authors to selectively enforce named arguments on specific APIs even in codebases that otherwise disable named argument calls.

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