I/6521: Add check for heading columns in merge cells command.

[jodator]

Suggested merge commit message (convention)

Other: Merge cells command should not merge cells from heading columns with other cells. Closes ckeditor/ckeditor5#6521.

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[coveralls]

Coverage remained the same at 100.0% when pulling fa958a1 on i/6521 into 0e25d38 on master.

[jodator]

I've found an undo bug: ckeditor/ckeditor5#6634. Also on master so a separate ticket.

[niegowski]

I would also add test like this:

it( 'should be false if selection has cells only from column headers and other cells - ???', () => {
    setData( model, modelTable( [
        [ '00[]', '01', '02', '03' ],
        [ '10', '11', '12', '13' ]
    ], { headingColumns: 2, headingRows: 1 } ) );

    tableSelection._setCellSelection(
        root.getNodeByPath( [ 0, 0, 0 ] ),
        root.getNodeByPath( [ 0, 1, 2 ] )
    );

    expect( command.isEnabled ).to.be.false;
} );

[niegowski]

I know that this is not the scope of this PR but getColumnIndexes confused me a little, maybe it should be sth like getFirstLastColumnIndexes? Same for getRowIndexes -> getFirstLastRowIndexes.

[jodator]

I know that this is not the scope of this PR

Yep, for me, it is outside a PR's scope. I'm fine with opening a new PR/issue for this. But here I didn't touch that code.

[jodator]

I would also add test like this:

Added this in 6c3ad1d  kinda redundant but why not.