[Bug]: sp-grid needs some sort of semantic structure

### Code of conduct

- [X] I agree to follow this project's code of conduct.

### Impacted component(s)

sp-grid

### Expected behavior

The grid component needs to create some sort of semantic structure to be comprehensible by assistive technologies.
We would typically recommend that it would be role="grid" with role="row" and role="gridcell" components beneath it. This will allow AT users to navigate the grid, get a count of the items and allow each cell to be encountered as a distinct unit.

Ideally this would have rows which match the visuals but we have generally accepted the semantics being a single column with all the rows containing a single cell. (https://react-spectrum.corp.adobe.com/components/GridView)

### Actual behavior

There is currently no semantic structure

### Screenshots

_No response_

### What browsers are you seeing the problem in?

Firefox, Chrome, Safari, Microsoft Edge

### How can we reproduce this issue?

1. Go to https://opensource.adobe.com/spectrum-web-components/storybook/index.html?path=/story/grid--default
2. Open Dev Tools in Chrome
3. Click on the "Switch to Accessibility TreeView" button
4. Note that there is no grid structure present

### Sample code that illustrates the problem

_No response_

### Logs taken while reproducing problem

_No response_

The issue identifies sp-grid and its Storybook default example; start by reproducing the problem in Chrome DevTools' Accessibility TreeView. Done means the component exposes a comprehensible grid structure with grid, row, and gridcell semantics, including distinct cells and navigable item counts for assistive technologies.

Written by the indexing model from the issue text.