fix(igxGrid): Fix unmerge when merge groups partially overlap.

[MayaKirova]

Closes #16358
Closes #16361

Please also re-test: #16146
And also do more extensive testing with more complex merge groups and integration scenarios with other features.

Additional information (check all that apply):

• Bug fix
• New functionality
• Documentation
• Demos
• CI/CD

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them

[mddragnev]

Also, there is an issue:

1. Open the performance sample with 1k records.
2. Add sorting expression

    public sortingExpressions: ISortingExpression[] = [
        { fieldName: 'Name', dir: SortingDirection.Asc },
    ];

3. Set cellMergeMode="always". Set [merge]=true on all columns
4. You get the following:

5. Click on the row with ID 534. You will get the following: The two rows above the unmerged row are broken

[mddragnev]

If you run the performance sample with 1k records with [cellMergeMode]="'always'" on the grid and [merge]="true" on the columns, then you get the following:

If you now click on row with ID = 8 and observe the row with ID 10 and 11 you can see that the group layout there is broken: