Support col[span] and pts in paste from excel

[Dumluregn]

Suggested merge commit message (convention)

Fix (table,paste-from-office): Tables pasted from MS Excel will have proper column widths. Closes #14521. Closes #14516.

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Additional information

See the detailed explanation what is going on here.

There's also another PR that should be merged after this one: https://github.com/cksource/ckeditor5-commercial/pull/5399.

[dufipl]

This case is still not working: #14521 (comment)

[illia-stv]

CI failing

[illia-stv]

For me looks good✅

[mlewand]

Good solution, just few minor remarks and we're good to go.

[mlewand]

LGTM, let's merge it once CI passes.

[niegowski]

This is not triggering column width normalization for such case:

<table>
    <colgroup>
        <col style="width: 30pt;">
        <col style="width: 70pt;">
    </colgroup>
    <tbody>
        <tr>
            <td>a</td>
            <td>b</td>
        </tr>
    </tbody>
</table>

And the final table ends up like this:

<figure class="table">
    <table class="ck-table-resized">
        <colgroup>
            <col style="width:30%;">
            <col style="width:70%;">
        </colgroup>
        <tbody>
        <tr>
            <td>a</td>
            <td>b</td>
        </tr>
        </tbody>
    </table>
</figure>

so pt values are treated as they would be %, the table gets full page width and the original sizes are not preserved.

The column width is normalized in post-fixer (but should be normalized before post-fixers are triggered).