Use PHPStan to enforce string literals in log messages

Problem/Motivation

Noted by @avpaderno in Slack:

I saw modules using code similar to \Drupal::logger($channel)->addError($e->getMessage()); despite the first argument for addError() and other similar logger methods is supposed to be a literal string.

In some cases it's possible that the exception message contains user input, and even if this isn't the case, the message sent to the logger should be a translatable string; any variable strings should be passed in as context.

Steps to reproduce

Proposed resolution

PHPStan supports a literal-string data type, we should use this everywhere that literal strings are expected which would notify users of the API that they are doing something incorrect.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet