Fix CodeAction for `cvc-complex-type.2.4.a` to add `elementFormDefault` to bound XSD

Given an XML document, foo.xml:

<foo xmlns="http://foo"
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="http://foo foo.xsd">
    <bar />
</foo>

and an XSD document, foo.xsd:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" targetNamespace="http://foo">
    <xs:element name="foo">
        <xs:complexType>
            <xs:sequence>
                <xs:element name="bar" />
            </xs:sequence>
        </xs:complexType>
    </xs:element>
</xs:schema>

the following error will appear:

with the following Quick Fix suggestion:

The CodeAction and Diagnostic for the error is currently incorrect/unsupported, and should add the elementFormDefault="qualified" attribute to the xs:schema tag in foo.xsd.

Reproduce the diagnostic and Quick Fix using the foo.xml and foo.xsd examples in the issue. Trace the CodeAction and Diagnostic handling for cvc-complex-type.2.4.a, then verify that the suggested change adds elementFormDefault="qualified" to the xs:schema tag in foo.xsd.