Add generate_svg function (closes #40)

[sruju333]

This PR is submitted as part of my GSoC 2026 application for the Gambit project (Idea 2: GameInterpreter).
It implements the generate_svg function as described in #40.

Changes:

1. Added generate_svg to src/draw_tree/core.py - generates a PDF first using generate_pdf, then converts to SVG using pdf2svg
2. Exported generate_svg from src/draw_tree/init.py
3. Updated README.md with SVG generation documentation, including install instructions for pdf2svg across macOS, Ubuntu and Windows

Requirements:

• pdflatex (same as generate_pdf)
• pdf2svg - install via brew install pdf2svg (macOS) or sudo apt install pdf2svg (Ubuntu)

Testing:
Tested locally with games/one_card_poker.ef - SVG generated and renders correctly in browser