Update ixdtf to handle unbound fraction length

[nekevss]

This PR updates ixdtf's fraction handling to parse fraction values beyond 9 digits in length per feedback from #6004.

It introduces a new enum Fraction in place of the previous nanosecond field that allows parsing fraction values beyond 9 digits while preserving the precision in the enum.

It also adds a variety of tests to test the new behavior while adjusting tests from the old behavior to the new behavior

[sffc]

Suggestion: change this to

pub struct Fraction {
    fraction_digits: u8,
    value: u64,
}

and then give it functions such as

impl Fraction {
    pub fn try_to_nanoseconds(self) -> Option<u64> {
        if self.fraction_digits <= 9 {
            // compute the pow and return
        } else {
            None
        }
    }
    pub fn to_truncated_nanoseconds(self) -> u64 {
        // compute pow, either + or -
    }
}

Another future advantage of this model is that the Fraction can directly return a FixedDecimal, which could make fractional second formatting more efficient in icu4x.

[robertbastian]

"Invalid" is not very descriptive. This is an error, that already tells me that something is not valid.

Suggested change

	InvalidFractionRange,
	ExcessiveSecondPrecision,

[nekevss]

I was double checking this and the error is specific to a Duration hour fraction and minute fraction potentially exceeding the range of u64::MAX when calculated. Maybe DurationFractionExceededRange (this would then be moved down into the Duration errors).