Fix RMC FixTime parsing that can lose sub-second precision with doubles

[justinswork]

When calling AddSeconds on the DateTimeOffset, in .NET 6 it rounds the double to the closest millisecond value. However, this changed in .NET 7. https://learn.microsoft.com/en-us/dotnet/core/compatibility/core-libraries/7.0/datetime-add-precision

Regardless of whether or not this package adds .NET 7 support, this change will ensure that we don't lose any subsecond precision from the NMEA message.

For NMEA messages that have less than 3 digits of precision, this will not change any behavior
"$GPRMC,141825.2,A,4249.92297,N,08548.52186,W,000.01,227.1,040322,005.5,W*54"

But for NMEA messages with more than 3 digits of precision, this will preserve that precision.
"$GPRMC,141825.1999,A,4249.92297,N,08548.52186,W,000.01,227.1,040322,005.5,W*54"

[justinswork]

#116

[justinswork]

@dotMorten we would appreciate this getting in soon so we can make the switch to .NET 8. Thanks!