Add support for inserting SVG images into documents.

Challenges

• Word uses a dual representation: SVG in mc:AlternateContent with a PNG/EMF fallback
• The SVG is stored as a separate part (image/svg+xml)
• The fallback raster image must be generated or provided by the caller

API Design

• document.add_picture(svg_path) — detect SVG, create both SVG part and fallback
• Options: provide explicit fallback image, or auto-generate using cairosvg/rsvg if available
• InlineShape.image should return the SVG data when the image is an SVG

XML

mc:AlternateContent > mc:Choice (with a16sv3:svgBlip) + mc:Fallback (with standard a:blip to raster)

Upstream: python-openxml#351, python-openxml#651, python-openxml#798 (12+8+6 = 26 comments)