[typescript][w-table] Typescript slot definitions for item-cell

[DerrikMilligan]

For a template section like this:

<w-table :headers="tableHeaders" :items="tableItems">
	<template #item-cell.name="{ item }">
		<td>Custom Cell...</td>
	</template>
</w-table>

Without this change the template slot #item-cell.name shows that it's an error.

I left the original item-cell without the slot modifier so that for autocompletion we still can see it and get the documentation for it.