Unicode: Try s/codepoint/code_point/g again

This time, without trailing 's'. Ran:

    git grep -l 'codepoint' | xargs sed -ie 's/codepoint/code_point/g

AK/StringBuilder.h

@@ -42,7 +42,7 @@ public:
     void append(const StringView&);
     void append(const Utf32View&);
     void append(char);
-    void append_codepoint(u32);
+    void append_code_point(u32);
     void append(const char*, size_t);
     void appendf(const char*, ...);
     void appendvf(const char*, va_list);