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/FileAPI/FileReader.idl

@@ -0,0 +1,35 @@
+#import <DOM/EventTarget.idl>
+#import <FileAPI/Blob.idl>
+
+// https://w3c.github.io/FileAPI/#dfn-filereader
+[Exposed=(Window,Worker)]
+interface FileReader : EventTarget {
+    constructor();
+    // async read methods
+    undefined readAsArrayBuffer(Blob blob);
+    undefined readAsBinaryString(Blob blob);
+    undefined readAsText(Blob blob, optional DOMString encoding);
+    undefined readAsDataURL(Blob blob);
+
+    undefined abort();
+
+    // states
+    const unsigned short EMPTY = 0;
+    const unsigned short LOADING = 1;
+    const unsigned short DONE = 2;
+
+    readonly attribute unsigned short readyState;
+
+    // File or Blob data
+    readonly attribute (DOMString or ArrayBuffer)? result;
+
+    readonly attribute DOMException? error;
+
+    // event handler content attributes
+    attribute EventHandler onloadstart;
+    attribute EventHandler onprogress;
+    attribute EventHandler onload;
+    attribute EventHandler onabort;
+    attribute EventHandler onerror;
+    attribute EventHandler onloadend;
+};