Add a static "show" method to NotificationElement

[Artur-]

Description

Enables use cases like

NotificationElement.show("The values have been stored in the database")

and

const undo = ...
NotificationElement.show(html`The values have been stored in the database <vaadin-button @click=${undo}>Undo</vaadin-button>`);

Fixes #914

Type of change

• Feature

Checklist

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

Additional for Feature type of change

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

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
1 Code Smell

No Coverage information
0.0% Duplication

[vaadin-bot]

This ticket/PR has been released with platform 22.0.0.