* Add support for autocompleting setting-keys in config-command
This PR implements the support for autocompletion of setting-keys in the
config-command and its related funtional tests. The implementation takes
existing command-line options like `--unset` or `--global` into account, as
well as existing exposable package-properties. The result is a context-
sensitive auto-completion of individual lists of setting-keys.
Additionally showing the value `suggest.` sub-keys has been allowed in
the config-command, as setting and unsetting them has alwas been possible,
and I did not want to distinguish between showing, setting and unsetting
those keys.
In the setting-key flattening-method I used the `array_is_list()` function,
which was added in PHP 8.1 recently. For backwards PHP-compatiblity the
package `symfony/polyfill-php81` has been added.
Signed-off-by: Stephan Jorek <stephan.jorek@brandung.de>
Jordi Boggiano