Move public headers into a separate include/woff2 directory

This is a first step to make WOFF2 a shared library. Public headers are moved
in their own directory to make the public API clearer. This is similar to
the c/include/brotli/ directory in Brotli. "using" commands are also removed
from the public headers, so that callers won't have unexpected side effect
when including the files.

src/woff2_dec.cc

@@ -6,7 +6,7 @@
 
 /* Library for converting WOFF2 format font files to their TTF versions. */
 
-#include "./woff2_dec.h"
+#include <woff2/decode.h>
 
 #include <stdlib.h>
 #include <algorithm>