Add an .editorconfig parser
Summary: Ref T5105. Add a PhutilEditorConfig class for parsing EditorConfig files. Depends on D11443 and D11444.
Test Plan: Wrote some unit tests.
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: johnny-bit, epriestley, Korvin
Maniphest Tasks: T5105
Differential Revision: https://secure.phabricator.com/D9678