LibWeb: Add an initial implementation for Web::FileAPI::FileReader

Some steps are still to be implemented, namely:
 * Properly aborting the read algorithm
 * Handling BinaryString type properly
 * Setting error on any error

But as it stands, this is enough functionality for the basic case of
reading the contents of a blob using the FileReader API.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -221,6 +221,7 @@ set(SOURCES
     FileAPI/BlobURLStore.cpp
     FileAPI/File.cpp
     FileAPI/FileList.cpp
+    FileAPI/FileReader.cpp
     FontCache.cpp
     Geometry/DOMMatrix.cpp
     Geometry/DOMMatrixReadOnly.cpp