DX: Add BlockValue constructors to force correct property editor alias and layout item type

[ronaldbarendse]

PR #16184 fixed the BlockValue implementation when serializing/deserializing, but creating a new value still required you to use the correct property editor alias and layout item type. For example, it was easy to get the following wrong:

var blockListValue = new BlockListValue()
{
    Layout =
    {
        // This won't store the layout, because it's not the correct one for BlockListValue
        [Constants.PropertyEditors.Aliases.BlockGrid] =
        [
            // Even if the type got updated to BlockGridValue, this layout item shouldn't be allowed
            new BlockListLayoutItem()
        ]
    }
};

Although the CMS codebase only manually creates these values in tests, users might want to programmatically create them. One use case is the Deploy artifact migrators, e.g. if you want to migrate Nested Content to Block List.

With this PR applied, you can use the new constructor overloads to specify the layout, which automatically uses the correct property editor alias (from the abstract property) and forces you to use the correct layout item type:

var blockListValue = new BlockListValue(
[
    new BlockListLayoutItem()
]);

[kjac]

Hi @ronaldbarendse --- this is good stuff!

I have taken the liberty to amend the PR a little with a few more pages from your book:

1. The GetLayouts overload on BlockValue that takes a propertyEditorAlias does not make any sense, because it is enforcing TLayout, so I have removed it (commit)
2. I have added an extra constructor on BlockItemData to simplify explicit/programmatic usages of that, e.g. in tests (commit).

Does it make sense to you?

[ronaldbarendse]

Does it make sense to you?

Yes, both changes make perfect sense, thanks! Feel free to merge when all tests have passed 🥳

[kjac]

👍 💯