allow translating English phrases in json domain output (#364)

resolves #274 by leveraging `sphinx.locale._` as a translation proxy.

Users will have to create .po files with the following phrases translated:
- "Required members"
- "Optional members"

sphinx_immaterial/apidoc/json/domain.py

@@ -40,6 +40,7 @@
 import sphinx.util.docutils
 import sphinx.util.logging
 import sphinx.util.matching
+from sphinx.locale import _
 import yaml  # pylint: disable=import-error
 
 from .. import object_description_options
@@ -864,7 +865,7 @@ def _render_related_type_list(related: Optional[List[str]], label: str) -> None:
                 ):
                     continue
                 heading = "%s members" % ("Required" if required else "Optional")
-                field_list, body = self._make_field(heading)
+                field_list, body = self._make_field(_(heading))
                 result.append(field_list)
                 for member_name, member_schema in properties.items():
                     if (member_name in required_properties) != required: