feat: add fontFamily, letterSpacing and lineHeight to text tools

[kuba-guzik]

Summary

Adds a fontFamily parameter to the create_text and set_text_content tools, allowing users to specify custom font families when creating or modifying text nodes in Figma.

Changes

• create_text (server.ts + code.js): Added optional fontFamily parameter (defaults to "Inter"). The Figma plugin attempts to load the requested font family and falls back to Inter if it's not available.
• set_text_content (server.ts + code.js): Added optional fontFamily and fontWeight parameters for changing the font of existing text nodes. If not provided, the existing font is preserved.
• TypeScript types: Updated the FigmaCommand type to include the new optional fields.

Motivation

Currently, all text created via create_text uses hardcoded "Inter" font family. This prevents users from building design systems that use custom fonts (e.g., brand typography like "Arbeit Pro", "Roboto", etc.).

Addresses #138.

API

// create_text - new optional parameter
fontFamily?: string  // e.g., "Roboto", "Arbeit Pro Trial" (default: "Inter")

// set_text_content - new optional parameters
fontFamily?: string  // change font family of existing text
fontWeight?: number  // change font weight of existing text

Fallback behavior

If the requested font is not available in Figma (not installed locally or not in the document), the plugin:

1. Logs a warning to the console
2. Falls back to "Inter" (for create_text) or keeps the existing font (for set_text_content)

This ensures backward compatibility — existing usage without fontFamily continues to work exactly as before.

Test plan

• Create text with default font (no fontFamily) — should use Inter as before
• Create text with fontFamily: "Roboto" — should use Roboto if available
• Create text with unavailable font — should fall back to Inter with console warning
• Use set_text_content with fontFamily to change an existing node's font
• Use set_text_content without fontFamily — should preserve existing font

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