Support approximate row number

[severo]

When showing data in HighTable, the exact position (ie. row number) is not always known, see source-cooperative/csv-table#10.

We could replace

hightable/src/helpers/dataframe/types.ts

Lines 52 to 55 in 1f7ec1b

	getRowNumber({ row, orderBy }: {
	row: number, // row index in the dataframe
	orderBy?: OrderBy
	}): ResolvedValue<number> | undefined

with

  getRowNumber({ row, orderBy }: {
    row: number, // row index in the dataframe
    orderBy?: OrderBy
  }): ResolvedValue<number> & { isEstimate?: boolean } | undefined

and add a data-is-estimate attribute on row number cells, so that estimated row numbers can be styled differently.