feat(multiselect): Add property to append current selection instead of replacing it

[nook24]

Defect Fixes

Currently when filtering a MultiSelect element, PrimeNG will truncate the currently selected elements when the "Select All" checkbox gets used.

Please see #18062 for a more detailed description of the behavior

Feature Requests

This PR introduces a new property appendPreviouslySelected to the multiselect component. By default, the value is false to keep the current behavior the default behavior.

When appendPreviouslySelected=true the "Select All" checkbox will append the options to to current selection.

This makes it possible to:

1. Select New York and Rome
2. Search for a use "Select All"
3. Istanbul and Paris will be appended to already selected New York and Rome

Please see this PR as an example implementation. Feel free to merge this or implement it by yourself to keep the API similar across all PrimeNG flavors.