feat: add vendor-option graphic-margin

[ger-benjamin]

⚠️ Draft PR

For #969

I add here a first VendorOption with the graphic-margin property. This is linked to the
additional Geostyler-styler GraphicFillPadding property: geostyler/geostyler-style#651

ToDo:

• Clarify questions
• Add tests
• Fix Fixmes

As It's the first VendorOption, I've several questions:

1. Is the code looking as expected ?
2. Can I add tests directly into src/SldStyleParser.geoserver.spec.ts ? Or should I had a dedicated test file ?
3. How can we add this new withVendorOption parameter into the Geostyler demo, cli or qgisparser ? Shopuld I create and issue about that ("be more flexible with parser options") ?