Simplify Grid definitions, with VerticalGrid and HorizontalGrid

## Description

MAUI simplifies stack layouts with `VerticalStackLayout` and `HorizontalStackLayout`, allowing for more concise, readable XAML.
The proposal here is to do something similar for the other common layout type: `Grid`, introducing `VerticalGrid` and `HorizontalGrid`.

A `VerticalGrid` is a list of rows, while a `HorizontalGrid` is a list of columns. They have these advantages over classic `Grid`:

- Allows grid definitions without a need to keep track of row/column indices - the numbering is implicit. Rows/columns can be most easily added/removed.
- Produces XAML markup that more closely matches the actual grid structure - each row/column is a separate indented group, making the grid easier to read and understand.

## Public API Changes

### Current Syntax
Here's a typical grid today:

```

```

### Proposed Syntax
The same grid as above with the proposed syntax is below:

```

```

Note that there's no longer a need to specify Grid.Row / Grid.Column, as the row/column index are computed automatically.

Occasionally, a grid is better conceptualized as a list of columns. In that case, it can be expressed with `HorizontalGrid`:

```

```

### Handling Row/Column spans

If a Row element has a child cell that spans adjacent columns, that's expressed like with a traditional `Grid`:
```

```

If a Row element has a child cell that spans rows below it, it's expressed as:
```

```

Using the `SpannedCell` dummy element helps makes the grid structure explicit - a cell would have been here, but instead its contents are provided by a row above. In both cases you look at a single Row and see how the children map to columns.

Similarly, an `EmptyCell` can be used for cases a grid cell is intentionally blank, without spanning:
```

```

The rules above mean that tooling can (ideally) enforce that every row/column has the expected number of children, helping minimize mistakes. Row/column indices aren't used with `VerticalGrid`/`HorizontalGrid`.

## Prior Art

* The Flutter Table widget is similar to `VerticalGrid`. It contains a list of `TableRow` elements. Flutter doesn't have an equivalent to `HorizontalGrid`.
* HTML tables are a list of `tr` (table row) elements. HTML developers should find this new Grid syntax intuitive, as it's the same model.
* Comet has [VGrid and HGrid](https://twitter.com/jtclancey/status/1478452660046995457), which is where I got the idea originally. Comet's syntax is more concise, as it omits the Row and Column elements, making them implicit based on item index. But I prefer the explicit Row/Column elements, especially for XAML, as it makes the grid structure more obvious and editing less error prone.

## Intended Use-Case

Just as `VerticalStackLayout` / `HorizontalStackLayout` will likely replace most uses of `StackLayout`, it's expected that most uses of `Grid` will move to using `VerticalGrid` / `HorizontalGrid` instead, given its advantages.

The issue names no files, tests, or implementation entry points. Start by locating the existing Grid, VerticalStackLayout, and HorizontalStackLayout APIs and their XAML behavior; done would require implementing and validating the proposed VerticalGrid and HorizontalGrid model, including spans, empty cells, and implicit indices.

Written by the indexing model from the issue text.