feat: theme property for individual menu items

[tltv]

Backported changes for Vaadin 14 from https://github.com/vaadin/web-components/pull/2401. Please read full description from there. Part of vaadin/flow-components#880.

Type of change

• [x] Feature

Checklist

• [x] I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• [x] I have added a description following the guideline.
• [x] The issue is created in the corresponding repository and I have referenced it.
• [x] I have added tests to ensure my change is effective and works as intended.
• [x] New and existing tests are passing locally with my change.
• [x] I have performed self-review and corrected misspellings.

Additional for Feature type of change

• [x] Enhancement / new feature was discussed in a corresponding GitHub issue and Acceptance Criteria were created.

[CLAassistant]

All committers have signed the CLA.