Add Figma Slides support to the official MCP server

The official Figma MCP server currently supports Design files and FigJam, but not Figma Slides. This is a significant gap.

Use case: We use Claude Code with the Figma MCP to generate designs programmatically. Being able to create and populate Figma Slides decks via MCP would be a game-changer for our agency workflow -- we produce dozens of presentations per month (pitches, case studies, client reports) and want to leverage AI to draft them directly in Figma Slides.

What's missing:

1. create_new_file only accepts "design" and "figjam" as editorType -- needs a "slides" option
2. use_figma / get_design_context should work with Slides files (reading and writing slide content, speaker notes, layouts)
3. Ideally: slide-specific primitives (add slide, reorder slides, set slide transitions)

Why it matters:

1. Figma Slides is positioned as the modern alternative to Google Slides / PowerPoint
2. AI-assisted presentation creation is one of the highest-demand use cases
3. Community projects like figma-slides-mcp and monorail-mcp already prove the demand -- but an official integration would be far more reliable and complete

Current workaround: Creating 1920x1080 frames in Design files and manually converting -- which defeats the purpose of Slides' native presentation features.

Would love to see this on the roadmap.