Add tests for TokenVisitor and SyntaxParser targets #148

Context

PR #147 introduced two new targets (TokenVisitor, SyntaxParser) that form the parsing infrastructure used by the skit executable. Currently, SyntaxKitTests only depends on SyntaxKit — there is no test coverage for the new modules.

Proposed Work

Add a SyntaxParserTests test target to Package.swift that covers at minimum:

• SyntaxParser.parse(code:) with a simple Swift snippet
• Basic TreeNode structure validation (id, parent, text, type)
• TokenVisitor traversal produces expected node count for a known input

This ensures regressions in the parsing pipeline are caught before they reach skit users.