Fix prop-type validation errors on Grid example

[juanca]

Closes https://github.com/juanca/react-aria-components/issues/60

• Implement failing test for missing props on Grid example
• Refactoring: ???

Note: Normally, I would prefer each test to also render an end-to-end Grid but it seems to be too noisy. I wonder if we can extrapolate this pattern?

[juanca]

Here is how the low-level API looks like now:

<DataGrid refs={[]}>
  <GridRow>
    <GridCell>{() => 'I am the'}</GridCell>
    <GridCell>{() => 'first row'}</GridCell>
  </GridRow>
  <GridRow>
    <GridCell>{() => 'I am the'}</GridCell>
    <GridCell>{() => 'second row'}</GridCell>
  </GridRow>
</DataGrid>

Here is how the low-level API would look like with render properties:

<DataGrid refs={[]}>
  {(activeRow, cellIndex) => (
    <GridRow active={activeRow} cellIndex={cellIndex}>
      {activeCell => <GridCell active={activeCell}>{() => 'I am the'}</GridCell>}
      {activeCell => <GridCell active={activeCell}>{() => 'first row'}</GridCell>}
    </GridRow>
  )}
  {(activeRow, cellIndex) => (
    <GridRow active={activeRow} cellIndex={cellIndex}>
      {activeCell => <GridCell active={activeCell}>{() => 'I am the'}</GridCell>}
      {activeCell => <GridCell active={activeCell}>{() => 'second row'}</GridCell>}
    </GridRow>
  )}
</DataGrid>

Here is how the low-level API could look like with context components:

<DataGrid refs={[]}>
  <GridRow index={0}>
    <GridCell index={0}>{() => 'I am the'}</GridCell>
    <GridCell index={1}>{() => 'first row'}</GridCell>
  </GridRow>
  <GridRow index={1}>
    <GridCell index={0}>{() => 'I am the'}</GridCell>
    <GridCell index={1}>{() => 'second row'}</GridCell>
  </GridRow>
</DataGrid>

[juanca]

Context-based solution looked the nicest but I foresee performance issues. If we completely avoid React.cloneElement then each cell will need to always ask if its row is active (Y-coordinate) and whether itself is active (X-coordinate). I am not sure how we can be lazy about the rendering since it is reacting to context changes.

[juanca]

@ahuth Let me know what you think about my above findings ^

I am hoping to extend usages of Cursor to other components that manage their own keyboard navigation.