Add tree-sitter-fortran to the package index

[LiamPattinson]

This PR adds tree-sitter-fortran to the package index.

Required

• Relevance: Tree-sitter is a programming language parsing framework. It is commonly used to build language-aware tools used by text editors such as vim, but may also be used for more general static analysis.
• Maturity: Tree-sitter itself is a very popular tool, with over 20000 stars on GitHub. The Fortran 90 grammar has over 40 stars, and is actively maintained.
• Availability: The source is freely available on GitHub.
• Open source: The package is licensed under MIT.
• Uniqueness: ✅
• README: Explains the preferred method of downloading and using the Tree-sitter CLI, and links to pages with more generic instructions.

Recommended

• Documentation: Limited documentation of the grammar itself, though Tree-sitter itself has extensive documentation.
• Contributing: CONTRIBUTING.md
• Tests: ✅