Add bilig

[gregkonush]

Adds bilig, a headless TypeScript spreadsheet engine for Node services and coding agents.

Notes:

• public repo: https://github.com/proompteng/bilig
• npm package: @bilig/headless
• this is listed as headless and vanilla because it exposes workbook/formula logic without a bundled UI

I kept UI-specific features false unless the package exposes the behavior directly.