feat: preserve newlines in XML attributes

[bshaffer]

I was able to test this via https://twigfiddle.com/s65ia4 and with my local checkout of phpdoc.

Essentially, this ensures that newlines are preserved in the XML attributes for structure.xml. Otherwise, the newline characters are lost.

Eg:

<method>
    <name>__construct</name>
    <docblock>
        <tag
            name="param"
            description="{
    Optional. Data for populating the Message object.

    @type string $name
          supply a name for this object
}"
            variable="options"
            type="array"
        />
    </docblock>
</method>

When usng SimpleXMLElement, this will be rendered without any newlines. E.g. { Optional. Data for populating the Message object. @type string $name supply a name for this object}.

By using the replace filter in twig, the result will instead be:

<method>
    <name>__construct</name>
    <docblock>
        <tag
            name="param"
            description="{&#10;    Optional. Data for populating the Message object.&#10;    @type string $name&#10;          supply a name for this object&#10;}"
            variable="options"
            type="array"
        />
    </docblock>
</method>

[jaapio]

This is nice! Thanks