Add --php-version to psalm?

[TysonAndre]

A common type of bug would be using features from newer PHP versions when the project needs to support older versions.

Obviously there's other ways to detect this, such as providing a plugin to run php --no-php-ini --syntax-check < file_or_tmpfile.php, or doing that with a separate tool

A reasonable default would be the php version used to run psalm (or guess from composer.json). For backwards compatibility, I'd assume 7.3 would be the most similar

Examples:

• Nullable types/void in PHP 7.1, object in php 7.2, etc
• Psalm does not warn about extra trailing commas in function calls (e.g. if running with PHP 7.1). Those became valid in PHP 7.3. (I'd assume PHP-Parser provides this)

<?php
call_user_func(/** @return void */function () {
    $other = 'x';
    var_export(is_string(
        $other,  // Psalm doesn't warn about trailing commas
    ));
    var_export(is_string($missing));  // Psalm correctly warns about this
});