feat(VDataTable): Screenreader suport for sorting

[viktorkratz]

Description

For screenreader users there was no possibility to dtermine wether a table column is sortable or not. The sort status was not readable too. This commit fixes this problem. In theory aria-sort can only be set to one table header (see https://www.w3.org/TR/wai-aria-1.2/#aria-sort), but in most screen readers it's even working with multiple columns containing aria-sort. For others it's only anouncing the first one, so i think setting it on every column which has an order set is the best option.

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