feat: add drag&drop support to Chart API

[bwajtr]

Description

This PR requires #6997 to be merged first.

This PR brings Drag&Drop support to Charts and complements this PR: vaadin/web-components#8488

There are three new events and event handlers. The support is enabled for now for Gantt chart via PlotOptionsGantt and many other charts if PlotOptionsSeries is used.

If this PR goes through, I can open another PR that would add the dragDrop property also other PlotOptions**** classes, where Drag and Drop is supported...

For now, the test has been made for Gantt and for Line chart.

Part of #4731

Type of change

• Bugfix
• Feature

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/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.
• I have not completed some of the steps above and my pull request can be closed immediately.

Additional for Feature type of change

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

[vaadin-bot]

TC Format Checker Report - 14:00 - 10 - Jan

There are 6 files with format errors

• To see a complete report of formatting issues, download the differences file

• To fix the build, please run mvn spotless:apply in your branch and commit the changes.

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  mvn spotless:apply
  

Here is the list of files with format issues in your PR:

vaadin-charts-flow-parent/vaadin-charts-flow-integration-tests/src/main/java/com/vaadin/flow/component/charts/examples/gantt/GanttInteractiveDemo.java
vaadin-charts-flow-parent/vaadin-charts-flow-integration-tests/src/main/java/com/vaadin/flow/component/charts/examples/lineandscatter/LineDragAndDrop.java
vaadin-charts-flow-parent/vaadin-charts-flow/src/main/java/com/vaadin/flow/component/charts/Chart.java
vaadin-charts-flow-parent/vaadin-charts-flow/src/main/java/com/vaadin/flow/component/charts/events/PointDragEvent.java
vaadin-charts-flow-parent/vaadin-charts-flow/src/main/java/com/vaadin/flow/component/charts/events/PointDragStartEvent.java
vaadin-charts-flow-parent/vaadin-charts-flow/src/main/java/com/vaadin/flow/component/charts/events/PointDropEvent.java