Use bignumber.js in internal Decimal Literal

[jitsedesmet]

The expression evaluator already uses bignumber.js to perform evaluations. However, this is not enough, an XSD decimal is unbounded, that means that after the calculations, we need to keep the value as a bignumber.

This means the DecimalLiteral typed value should be a bignumber in order to be spec compliant.

This is originally mentioned in an issue of the sparqlee repo: comunica/sparqlee#45.