feat: draw quivers with GraphViz via to_dot and a package extension

[pbelmans]

What

Adds quiver visualization to QuiverTools. Parallel arrows and loops (a quiver's defining features) are drawn as themselves rather than collapsed to a single labelled edge.

API

• to_dot(Q) returns a Graphviz DOT string. It is dependency-free and lives in the core package: one i -> j line per arrow, self-loops spread around their vertex with compass ports, and isolated vertices emitted explicitly.
• using GraphViz activates a package extension that makes a Quiver display as an SVG drawing inline in notebooks (Pluto, IJulia) and VS Code.
• draw(Q), from the REPL, renders the quiver and opens it in the system's default viewer, returning the temporary file path.

Design

• The extension (ext/QuiverToolsGraphVizExt.jl) mirrors the existing Oscar extension. GraphViz is a weak dependency, so a plain using QuiverTools never loads a graphics binary; to_dot is always available, while the SVG show method and draw activate only once GraphViz is loaded.
• Graphviz_jll is pinned to >= 2.50. Without the pin the resolver can select an older build whose libgvc binding is undefined, which crashes using GraphViz with a cryptic error.

Tests and docs

• A to_dot testset covers arrow bookkeeping, port-spaced loops, and the no-arrow (isolated vertices) edge case; the exact output is locked by doctests.
• Docstrings for to_dot and draw are added to the Quivers manual page.