Feature suggestion: allow for "standards" to be hidden

[jrfnl]

Repost from squizlabs/PHP_CodeSniffer#2760:

Use-case: sniffs in generic sniff collections - like the Generic set in PHPCS - can be freely used by other standards, but shouldn't ever be included in a custom ruleset like <rule ref="Generic"/> as they may (and often will, like Generic) contain sniffs which contradict each other.

For the build-in Generic standard, PHPCS makes a hard-coded exception and doesn't list it when someone calls up phpcs -i.

https://github.com/squizlabs/PHP_CodeSniffer/blob/120c713213260a38988ac17816058ba3686f4e8f/src/Util/Standards.php#L107-L110

https://github.com/squizlabs/PHP_CodeSniffer/blob/120c713213260a38988ac17816058ba3686f4e8f/src/Util/Standards.php#L197-L200

I'd like to propose making that a feature available to external standards as well.
For one, I imagine the SlevomatCodingStandard could use it, but I see some more use-cases in the future.

I envision this can most easily be done by adding a new attribute to the ruleset element in the xsd and adjusting the code for the -i option to respect the setting found there.

Something like list with the default being true.

<ruleset xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    name="Generic"
    list="false"
    xsi:noNamespaceSchemaLocation="../../../phpcs.xsd">

Implementation-wise, AFAICS this will be easiest to implement via the Standards::getInstalledStandardDetails() method and the Standards::getInstalledStandards() should then probably defer to that method instead of it being a stand-alone method.

Opinions ?

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Thinking about this issue now, I have a feeling end-users might get confused if they don't see an expected standard when running phpcs -i, so hiding the standard(s) may not be the right action.

Possibly listing them as a separate category "Additional non-standard rulesets" or something like that ? (including Generic)
Or listing them with an * next to their name or something, might be better.

I'd love to see more input from end-users about what would be most helpful to them.

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