AK: Improve the API of StringBuilder

Author: https://github.com/f-eiwu
Commit: d8df9c510c
Pull-request: https://github.com/SerenityOS/serenity/pull/1386
Reviewed-by: https://github.com/awesomekling

AK/StringBuilder.h

@@ -45,9 +45,9 @@ public:
     void appendf(const char*, ...);
     void appendvf(const char*, va_list);
 
-    String build();
-    String to_string();
-    ByteBuffer to_byte_buffer();
+    String build() const;
+    String to_string() const;
+    ByteBuffer to_byte_buffer() const;
 
     StringView string_view() const;
     void clear();