[BUG] Grammar missing support for the readonly keyword

[mmanela]

Describe the bug

The grammar does not properly parse the readonly keyword

Requirements

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.

To Reproduce

Grammar does not parse the readonly keyword properly leading to errors in the parse tree.

public static readonly int $x = 0;

Which leads to a tree with an error

(property_declaration
        (visibility_modifier)
        (static_modifier) 
        type: (type_specifier
          (qualified_identifier
            (identifier)))
        (ERROR)
        (property_declarator
          name: (variable)
          value: (integer)))

Expected behavior

I expect the readonly keyword to parse properly