Grid Cell merging

[MayaKirova]

Grid cell merging feature as per spec

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

[ChronosSF]

@dkamburov , @MayaKirova , I concluded testing the scenarios that I intended . Putting this as verified as I prefer the bulk to be merged and the items I found be handled as separate prs but you can go on about it as you wish.

I'll be on PTO until Tuesday so you can have someone else verify fixes if they become available prior.

[MayaKirova]

@dkamburov , @MayaKirova , I concluded testing the scenarios that I intended . Putting this as verified as I prefer the bulk to be merged and the items I found be handled as separate prs but you can go on about it as you wish.

I'll be on PTO until Tuesday so you can have someone else verify fixes if they become available prior.

@ChronosSF @dkamburov Sure, I can add fixes in separate PRs. Just a side note about the one with master-detail:

This is expected, because activation and/or selection is on the collapsed cell. And as per the spec activation/selection break the merge sequence.

All additional fixes can be found here and can be tested/merged separately: #16107

[simeonoff]

Why is important needed here and in the consequent placeholder selectors?

[MayaKirova]

It's because if the pinned cell style:

    %grid-cell--pinned {
        position: relative;
        background: inherit;
        z-index: 9999;
    }

It overwrites the background, when the cell is also pinned.