Issue #3477377 by benabaird: Add a search filter to restrict items in the dropdown

.cspell-project-words.txt

config/schema/multiselect_dropdown.schema.yml

js/dist/multiselect-dropdown-views.js

js/dist/multiselect-dropdown.js

js/dist/multiselect-dropdowns.js

js/src/multiselect-dropdown-views.js

js/src/multiselect-dropdown.js

js/src/multiselect-dropdowns.js

...own_bef/config/schema/multiselect_dropdown_bef.schema.yml

...multiselect_dropdown_bef/multiselect_dropdown_bef.install

...xposed_filters/filter/MultiselectDropdownFilterWidget.php

multiselect_dropdown.install

multiselect_dropdown.module

phpstan-baseline.neon

src/Element/MultiselectDropdown.php

src/Plugin/Field/FieldWidget/MultiselectDropdownWidget.php

templates/multiselect-dropdown.html.twig

...nfig/install/views.view.multiselect_dropdown_bef_test.yml

...ig/install/core.entity_form_display.node.node.default.yml

...ct_dropdown_test/src/Form/MultiselectDropdownTestForm.php