Add a linter rule for declaring nested functions

Summary: Ref T7409. Adds ArcanistXHPASTLinter::LINT_INNER_FUNCTION to advise against declaring PHP functions within an outer function. Based on Squiz_Sniffs_PHP_InnerFunctionsSniff.

Test Plan: Added unit tests.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7409

Differential Revision: https://secure.phabricator.com/D12389