Allow using readonly as function name

Don't treat "readonly" as a keyword if followed by "(". This
allows using it as a global function name. In particular, this
function has been us

Allow using readonly as function name

Don't treat "readonly" as a keyword if followed by "(". This
allows using it as a global function name. In particular, this
function has been used by WordPress.

This does not allow other uses of "readonly", in particular it
cannot be used as a class name, unlike "enum". The reason is that
we'd still have to recognize it as a keyword when using in a type
position:

class Test {
public ReadOnly $foo;
}

This should now be interpreted as a readonly property, not as a
read-write property with type `ReadOnly`. As such, a class with
name `ReadOnly`, while unambiguous in most other circumstances,
would not be usable as property or parameter type. For that
reason, we do not support it at all.

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