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/convert_woff2ttf_fuzzer_new_entry.cc

@@ -1,5 +1,5 @@
 #include <string>
-#include "woff2_dec.h"
+#include <woff2/decode.h>
 
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t data_size) {
   // Decode using newer entry pattern.