feat: parse schematic text font sizes

[vahapogut]

Fixes #11

Summary

• Parse schematic_text.font_size with the shared distance parser.
• Preserve the existing 0.18mm default output value.
• Add regression tests for distance string font sizes and the default.

Verification

• bun test tests\schematic-text.test.ts
• bun x biome check src\schematic\schematic_text.ts tests\schematic-text.test.ts
• bun run build
• git diff --check