Add length field to Identifier struct and define String struct in lexer header

SLS_C/include/sls/lexer.h

@@ -34,6 +34,7 @@ typedef enum {
 
 typedef struct {
     const char *name;
+    size_t length;
     uint8_t is_literal;
 } Identifier;
 
@@ -42,4 +43,9 @@ typedef struct {
     IntegerBuiltInType type;
 } IntegerLiteral;
 
+typedef struct {
+    const char *value;
+    size_t length;
+} String;
+
 #endif // SLS_LEXER_H