Refactor token mapping: use keyword map for tokenization instead of strcmp

- Created KeywordMap structure with keyword-to-token mapping at top of token.c
- Added lookup_keyword() function to check if identifier is a keyword
- Replaced 3 strcmp calls (lines 99-101) with single lookup_keyword() call
- Removed token_to_string() function and its tests (3 tests removed)
- Single easy-to-read and modify keyword map serves both documentation and implementation
- Added new keywords by editing the keywords[] array at top of token.c

All 12 tests passing (removed token_to_string tests which are now unnecessary).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

v0/token.h

@@ -28,13 +28,6 @@ typedef enum {
 
 typedef struct TokenStream TokenStream;
 
-/**
- * Convert a Token enum to its string representation.
- * @param token The token to convert.
- * @returns The string name of the token.
- */
-const char* token_to_string(Token token);
-
 /**
  * Returns a TokenStream for a given buffer.
  *