The `closeOnSelect` option was previously used to control whether
or not the dropdown was closed when an option was selected. This
could be simulated by triggering the `open` event after the `close`
event was received, but it makes sense to abstract it out into a
decorator.
This also adds support for not closing the dropdown when the control
key is being held. This is useful when multiple options need to be
selected in quick succession, so the dropdown does not have to be
reopened.
This also adds documentation that covers both changes.
This closes https://github.com/select2/select2/pull/2735.
This closes https://github.com/select2/select2/issues/3017.
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