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