Closes#1951
Gutter markers only recognised a short alias list and the runtime truncated icon references at 32 bytes, so ordinary icon names and package paths were dropped. Markers now accept any built-in icon name, and API 1.3 `contributes.markerIcons` declares package SVGs (with an optional dark variant) that are size-limited, sanitised and rendered through the icon registry. The shared SVG sanitiser now also rejects URL strings in any CSS function (e.g. image-set), not only url(). Verified on macOS in light and dark themes.
Co-authored-by: arnaud-ritti <[email protected]>