src: flatten ContextifyContext

Flattens ContextifyContext allows the context interface to be used in
other parts of the code base.

PR-URL: #17560
Reviewed-By: Michaël Zasso <[email protected]>
Reviewed-By: Tiancheng "Timothy" Gu <[email protected]>

Author: devsnek

node.gyp

@@ -351,6 +351,7 @@
         'src/node.h',
         'src/node_buffer.h',
         'src/node_constants.h',
+        'src/node_contextify.h',
         'src/node_debug_options.h',
         'src/node_file.h',
         'src/node_http2.h',