feat: Add generate_svg implementation using pdf2svg

[Ao-chuba]

@edwardchalstrey1 ,
This PR implements the requested generate_svg feature

• Created the generate_svg function locally inside src/draw_tree/core.py. It mirrors the existing generate_png approach by running
• generate_pdf first, then utilizing subprocess.run to convert the PDF into a vector SVG format using pdf2svg.
• Exposed a --svg option to the CLI via cli.py and exported generate_svg as a public module method in init .py.
• Updated README.md
• Fixed a minor bug in generate_png where it incorrectly instructed generate_pdf to write the intermediate PDF file to the destination path rather than the requested temporary directory, which was sporadically causing FileNotFoundError during Ghostscript/Imagemagick conversions.
• Added TestSvgGeneration mapping coverage for defaults, missing files, and uninstalled binaries. Ensured that pytest successfully completes across the test suite locally.

closes #40