AK: Use an enum to specify the open mode instead of a bool

Let's replace this bool with an `enum class` in order to enhance
readability. This is done by repurposing `MappedFile`'s `OpenMode` into
a shared `enum` simply called `Mode`.

AK/MemoryStream.cpp

@@ -12,9 +12,9 @@
 
 namespace AK {
 
-FixedMemoryStream::FixedMemoryStream(Bytes bytes, bool writing_enabled)
+FixedMemoryStream::FixedMemoryStream(Bytes bytes, Mode mode)
     : m_bytes(bytes)
-    , m_writing_enabled(writing_enabled)
+    , m_writing_enabled(mode == Mode::ReadWrite)
 {
 }