Add a linter rule for default parameters

Summary: Ref T7409. Adds ArcanistXHPASTLinter::LINT_DEFAULT_PARAMETERS for ensuring function/method parameters with a default value are declared after those without a default value. Based on PEAR_Sniffs_Functions_ValidDefaultValueSniff.

Test Plan: Added test cases.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7409

Differential Revision: https://secure.phabricator.com/D12418