Support resvg as alternative to rsvg-convert for SVG rendering

Installing rsvg-convert on Windows can be challenging for users, requiring either manual binary installation or complex package manager setup. This affects workflows that convert SVG to PDF/LaTeX output, including mermaid diagrams with mermaid-format: svg.

resvg (https://github.com/linebender/resvg) is a modern Rust-based SVG rendering library with better cross-platform support. It provides pre-built binaries for all platforms and is available through Windows package managers (scoop, winget), making installation significantly easier for Windows users.

We could add resvg as an alternative SVG converter alongside rsvg-convert and Inkscape. The conversion pipeline in src/resources/filters/quarto-post/pdf-images.lua already handles rsvg-convert and has infrastructure for tool detection.

https://github.com/quarto-dev/quarto-cli/blob/c9008d7cb1d088f6837fc41ed1f138b8f443a9d1/src/resources/filters/quarto-post/pdf-images.lua#L28-L61

We could implement this similar to the existing use-rsvg-convert option, perhaps with a more general svg-converter option that accepts resvg, rsvg-convert, or inkscape, with automatic fallback to whichever tool is available.

This came up while investigating #13661, where better SVG tooling options would help Windows users. Directly addresses the request in #1144 for specifying alternative SVG conversion tools.