Add Swift Package Manager support

[nhubbard]

This pull request is intended to add Swift Package Manager support so that we can use this package with the wonderful https://github.com/ChimeHQ/SwiftTreeSitter without manually adding another build step to our Swift projects.

As an example, here's how it would be used:

1. Add this package as a dependency to your Xcode project or Package.swift file.
2. Use it as follows:

   import SwiftTreeSitter
   import TreeSitterLatex
   
   let language = Language(language: tree_sitter_latex())
   let parser = Parser()
   try parser.setLanguage(language)
   
   let source = "" // Insert LaTeX source code here
   let tree = parser.parse(source)!
   // Use the tree as you see fit!

The work for this PR is based on tree-sitter/tree-sitter-java#113. I've tested it with the included test document and a few LaTeX documents I've written myself and it worked perfectly.

[pfoerster]

@nhubbard Thanks!