`where` output on Windows may contain multiple paths

[jrfnl]

As became clear when testing PR #1105, running shell_exec() with the Windows where command may return multiple paths.

Also see: #1105 (review)

Once #1105 will be merged, there will be three places in the codebase which use where/which to determine whether a CLI tool is available to be used and all three do so in a slightly different manner.

It may be worth creating a generic utility function in the Util\Common class to do command line tool path determination (correctly) and use that utility in those three places.

I imagine, the signature of the new method could look something like this:

public static function findExecutable(string $cmd): string|false

Places in the codebase where this type of path determination is done:

• PHP_CodeSniffer\Config::getExecutablePath()
• PHP_CodeSniffer\Util\Common::isStdinATTY()
• (Once Skip tests when 'git' command is not available #1105 is merged) PHP_CodeSniffer\Tests\Core\Filters\AbstractFilterTestCase::checkGitCommandAvailable()

Also possibly relevant for #1107 ?

[jrfnl]

For the record: #1105 will be merged without introducing new which/where logic. All the same, I believe it may still be worth introducing the suggested method. Low priority though.