Support nested <svg> elements

[JSUYA]

[Example]

<svg width="64" height="64" viewBox="0 0 64 64" xmlns="http://www.w3.org/2000/svg" fill="none">
    <rect rx="8" fill="#80ff45" width="64" height="64"/>
    
    <svg x="12.5%" y="37.5%" width="50%" height="32" viewBox="0 0 32 32" xmlns="http://www.w3.org/2000/svg">
        <rect rx="4" fill="#ff4596" width="32" height="50%"/>
    </svg>

</svg>

Our SVG loader parses the <svg> element at the check of validation(header()) and skips any <svg> elements it encounters thereafter.
We can make loader->doc an array type and make the first doc node the main doc node.
And every node can store and reference its parent <svg> element node.
This work should be process in conjunction with valid check function. it may require more time or may not proceed according to our policy.