LibDiff: Add Diff::apply_patch

Given a set of lines from the file we are patching, and a patch itself,
this function will try and locate where in the file to apply that patch,
and write the result of patching that file (if successful) to the output
stream.

Userland/Libraries/LibDiff/Hunks.h

@@ -71,6 +71,11 @@ struct Header {
     String new_file_path;
 };
 
+struct Patch {
+    Header header;
+    Vector<Hunk> hunks;
+};
+
 class Parser : public GenericLexer {
 public:
     using GenericLexer::GenericLexer;