Make the source argument for setDataAtCell in the Merge Cells plugin more specific.

[jansiegel]

Context

The Merge Cells plugin utilizes populateFromArray and setDataAtCell methods to fill the cells under the merged cells with nulls.

Currently, the source argument given in the beforeChange/afterChange hooks differed between the following use cases:

• When the merged cells were created at initialization, the source was defined as edit (used to be populateFromArray).
• When the merged cells were created after initialization, the source was defined as MergeCells.

After the change introduced in this PR, both cases will be passing the source argument as MergeCells.

I think in both cases, the source should be more specific, like MergeCells.clearCells, but the change could be considered breaking, so I think we should maybe introduce this change with the next major release. WDYT @evanSe?

How has this been tested?

Added a test case.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature or improvement (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Additional language file or change to the existing one (translations)

Related issue(s):

1. handsontable/dev-handsontable#1763
2. handsontable/dev-handsontable#1764

Affected project(s):

• handsontable
• @handsontable/angular
• @handsontable/react
• @handsontable/vue
• @handsontable/vue3

Checklist:

• I have reviewed the guidelines about Contributing to Handsontable and I confirm that my code follows the code style of this project.
• I have signed the Contributor License Agreement
• My change requires a change to the documentation.

[github-actions]

Launch the local version of documentation by running:

npm run docs:review df79d62832de5c11005e4c7f27e4a52407435cba