Optionally disallow access modifier override on extensions

[masters3d]

Instead of

private extension SomeType {
    func someFuncA() {}
}

Prefer

extension SomeType {
    fileprivate func someFuncA() {}
}

[masters3d]

Find all the occurrences of extension in the structure:
"key.kind" : "source.lang.swift.decl.extension"
then we could find out if there is a syntax type before the extension:
"type" : "source.lang.swift.syntaxtype.attribute.builtin"
then we check if the built in attribute is any of the access modifier;
If true, then we can mark the violation.