feat: add keepFilter option

[sissbruecker]

Adds a keepFilter option to <vaadin-multi-select-combo-box> that allows to keep the filter value after selecting an item.

Some notes regarding the behavior:

• Filter is preserved when selecting items using either mouse or keyboard
• Input value is restored to the filter after selecting items using keyboard. Selecting items using keyboard overrides the input value with the item label so this needs to be reverted.
• The filter is cleared when:
  • Closing the overlay either due to focus out or canceling input with Escape
  • Adding a custom value
  • Trying to commit a non-existing value. Might be better UX to keep the filter, but I left it out due to complexity, and it seems like an edge-case.

Closes #5930

[web-padawan]

Tested manually and the feature works as expected 👍

The only problem I found is related to clear button on mobile but that's not related to this PR as the issue can be reproduced on main branch too. Created #7066 for it.

[sonarcloud]

Quality Gate passed

Kudos, no new issues were introduced!

0 New issues
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud