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.cpp

@@ -68,9 +68,19 @@ ByteBuffer StringBuilder::to_byte_buffer()
 String StringBuilder::to_string()
 {
     auto string = String((const char*)m_buffer.pointer(), m_length);
-    m_buffer.clear();
-    m_length = 0;
+    clear();
     return string;
 }
 
+StringView StringBuilder::string_view() const
+{
+    return StringView { (const char*)m_buffer.pointer(), m_length };
+}
+
+void StringBuilder::clear()
+{
+    m_buffer.clear();
+    m_length = 0;
+}
+
 }

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);