docs: Add F# grammar implementation status tracking

[NatElkins]

Summary

This PR adds a comprehensive IMPLEMENTATION_STATUS.md document that tracks the current state of F# language feature support in the tree-sitter grammar.

Motivation

Currently, it's difficult for contributors to understand:

• Which F# features are already implemented
• Where to find tests for existing features
• What features are missing and need work
• The relative priority of unimplemented features

What this adds

The document provides:

• ✅ Complete mapping of 100+ F# language features against the F# Language Specification
• 📍 Test file references for verification of existing features
• 🎯 Priority rankings based on real-world usage patterns
• 📊 Clear status indicators (✅ Implemented, 🚧 Partial, ❌ Not Implemented)

How this helps

1. For contributors: Clear roadmap of what needs implementation
2. For users: Transparency about current grammar limitations
3. For maintainers: Tracking document to update as features are added

Example entries

| Wildcard slice expressions `arr.[*]` | ✅ | `expr.txt:3785` | |
| Sequence expressions `seq { ... }` | 🚧 | `expr.txt:847` | Only basic form, missing yield/for/while |
| Units of measure | ❌ | | `float<kg>`, `int<m/s>` |

This is the first of several planned PRs to improve F# grammar coverage. Future PRs will tackle the high-priority missing features identified in this document.