AK: Add StringBuilder::string_view() and StringBuilder::clear()

The former allows you to inspect the string while it's being built.
It's an explicit method rather than `operator StringView()` because
you must remember you can only look at it in between modifications;
appending to the StringBuilder invalidates the StringView.

The latter lets you clear the state of a StringBuilder explicitly, to
start from an empty string again.

AK/StringBuilder.h

@@ -24,6 +24,9 @@ public:
     String to_string();
     ByteBuffer to_byte_buffer();
 
+    StringView string_view() const;
+    void clear();
+
 private:
     void will_append(int);