refactor!: make appendToken static

`appendToken` is a internal function to allocate and append a new token to the current token, which is the end of the token list. This change can prevent misuse of the function.

BREAKING CHANGE: `appendToken` is no longer declared in the header or with external linkage.

Author: lumynou5

include/tokenizer.h

@@ -51,5 +51,3 @@ const char* tokenize(const char* src, Token** res);
 ///
 /// \param tk The most parent token to free.
 void freeTokens(Token* tk);
-
-void appendToken(Token** curr, TokenKind kind, const char* str, unsigned int num);

src/tokenizer.c

@@ -26,6 +26,8 @@
     } \
 }
 
+static void appendToken(Token** curr, TokenKind kind, const char* str, unsigned int num);
+
 const char* tokenize(const char* src, Token** res) {
     Token head;
     head.next = NULL;