Everywhere: Rename to_{string => deprecated_string}() where applicable

This will make it easier to support both string types at the same time
while we convert code, and tracking down remaining uses.

One big exception is Value::to_string() in LibJS, where the name is
dictated by the ToString AO.

Userland/Libraries/LibWeb/CSS/Parser/Function.cpp

@@ -18,7 +18,7 @@ Function::Function(FlyString name, Vector<ComponentValue>&& values)
 
 Function::~Function() = default;
 
-DeprecatedString Function::to_string() const
+DeprecatedString Function::to_deprecated_string() const
 {
     StringBuilder builder;
 
@@ -27,7 +27,7 @@ DeprecatedString Function::to_string() const
     builder.join(' ', m_values);
     builder.append(')');
 
-    return builder.to_string();
+    return builder.to_deprecated_string();
 }
 
 }