Call out @Migrate as a khan-specific directive

This audit moves the `@migrate` directive to a separate, commented
section so that it's clear that the directive is khan-specific. At some
point we'd like to discover khan-specific directives automatically.

Issue: see rKLefcb6f0fd2

Test plan:
ka-lint services/*/*.graphql in KA root

Auditors: csilvers

linters.py

@@ -857,11 +857,13 @@ def process(self, f, contents_of_f):
         contents_of_f += """
         scalar _FieldSet
         directive @external on FIELD_DEFINITION
-        directive @migrate(from: String!, state: String!) on FIELD_DEFINITION
         directive @requires(fields: _FieldSet!) on FIELD_DEFINITION
         directive @provides(fields: _FieldSet!) on FIELD_DEFINITION
         directive @key(fields: _FieldSet!) on OBJECT | INTERFACE
         directive @extends on OBJECT | INTERFACE
+
+        # Khan-specific directives
+        directive @migrate(from: String!, state: String!) on FIELD_DEFINITION
         """
 
         schema_files = glob.glob(os.path.join(os.path.dirname(f), '*.graphql'))