Add a lint check for deprecated argument order to "implode()"

Summary:
Ref T13428. Historically, "implode()" accepts arguments in either order. PHP 7.4+ warns about glue not being first.

Add a lint check when the second parameter is a static scalar, implying it is the glue parameter.

Test Plan: Ran unit tests. See next change.

Maniphest Tasks: T13428

Differential Revision: https://secure.phabricator.com/D20857