Add a linter rule for lowercase function names

Summary: Ref T7409. Adds ArcanistXHPASTLinter::LINT_LOWERCASE_FUNCTIONS to ensure that builtin PHP functions are lowercased. Based on Squiz_Sniffs_PHP_LowercasePHPFunctionsSniff.

Test Plan: Added unit tests.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7409

Differential Revision: https://secure.phabricator.com/D12377