Add basic <symbol> support in SVG

[Theo1335]

Add basic support in SVG parsing

This PR adds support for SVG <symbol> elements.

• Reuse existing group parsing logic for symbols.
• Add a unit test covering symbol parsing and cross-reference registration.

Checklist:

• A unit test is covering the code added / modified by this PR

• In case of a new feature, docstrings have been added, with also some documentation in the docs/ folder (N/A)

• A mention of the change is present in CHANGELOG.md

• This PR is ready to be merged

By submitting this pull request, I confirm that my contribution is made under the terms of the GNU LGPL 3.0 license.

[andersonhc]

Thank you for this PR @Theo1335 , it's looking very promising!

I found one problem testing this SVG

The symbols are rendered but the scale is wrong.

Can you please check how to fix this?

[Theo1335]

Thank you for your feedback @andersonhc !

I think I've finally solved the scaling issue with <symbol> elements: the viewBox is now extracted and stored when <symbol> is parsed, and when a <use> directive references a symbol with a width and height, the appropriate scaling is applied based on the `viewBox's dimensions.

Please let me know what you think.

[andersonhc]

Right now the test is failing:

    @force_nodocument
    def build_symbol(self, symbol: "Element") -> GraphicsContext:
        """Parse <symbol> as reusable content, not rendered directly."""
        group = self.build_group(symbol)
        viewbox = symbol.attrib.get("viewBox")
        if viewbox:
            parts = viewbox.replace(",", " ").split()
            if len(parts) >= 4:
>               setattr(group, "_vw", float(parts[2]))
E               AttributeError: 'GraphicsContext' object has no attribute '_vw'

I don't think adding the viewbox attribute to the GraphicsContext is the best approach.
I recommend looking into using the GaphicsContext's transform to apply the viewbox.

Apply translate(-vx, -vy) @ scale(1 / vw, 1 / vh) to the symbol group should make it normalized to unit coordinates. In build_xref(), apply scale(width, height) @ translate(x, y).