feat(sort): sort on a value returned by the column

[mihir-kandoi]

Sorting compares cell.content, so a column that renders something other than its stored value sorts by the value the user cannot see. In Frappe this shows up on Link columns that display a document's title field: the cell reads Acme Corp but sorts under CUST-0099.

compareValue already solves this for filtering. sortValue is the same idea for sorting: a column with a sortValue(cell) function sorts on what that returns, and columns without one are unchanged.

{
    name: "Customer",
    sortValue: cell => cell.title || cell.content,
}