Compact decimal should retain 2 significant digits for small numbers

[sffc]

This was found from @sven-oly's conformance test suite.

If the input to CompactDecimalFormatter::format_fixed_decimal is a fractional number less than 1, we should retain 2 significant digits according to the ECMA-402 specification (citation available upon request). This is highlighted in the conformance test suite. Currently we are just rounding off the trailing fractional digits completely.

icu4x/experimental/compactdecimal/src/compactdecimal.rs

Line 492 in 582e46c

pub fn format_fixed_decimal(&self, value: FixedDecimal) -> FormattedCompactDecimal<'_> {