Add missing superglobals

[sirbrillig]

This adds support for some superglobals which were previously missing:

• $http_response_header
• $HTTP_RAW_POST_DATA
• $php_errormsg

Fixes #176

[jrfnl]

@sirbrillig You may find the helper functions in the PHPCSUtils Variables class useful for this, particularly Variables::isSuperglobal()/Variables::isSuperglobalName().

https://phpcsutils.com/phpdoc/classes/PHPCSUtils-Utils-Variables.html

Also note that $php_errormsg is not a superglobal. It is only available within the scope in which the error occurred and only if the track_errors configuration option is turned on (it defaults to off).

See: https://www.php.net/manual/en/reserved.variables.phperrormsg.php

[jrfnl]

Oh and I forgot to mention Variables::isPHPReservedVarName()

[sirbrillig]

Thanks! I'll make another PR to fix that up. Judging by the docs, I think I should just remove php_errormsg entirely from the checks because it's deprecated and only available in specific circumstances. (I wish the PHP superglobals docs page would note that...).