Support "toFixed"

[danielvillalbamota]

Currently expression round works rounding to integer and decimal.

According to other usages of jexl (javascript) we have toFixed to round decimals, so we could also implement toFixed as

super().add_transform("toFixed", lambda x, decimals: round(x, decimals)).

Moreover, should we limit the usage of round expression to round integer? This would be using the python function "round" without using the second parameter (ndigits) https://docs.python.org/3/library/functions.html#round

[fgalan]

If we change existing transformation we have to analyse first its current usage, to detect potential backward compatibility problems.