Add support for Fixed Width files

[TheBrambleShark]

Fixes serdedotnet/serde#276
Closes serdedotnet/serde#281

Sorry for the delay on this. I've mostly only had time to work on this while at work and things got busy. I changed my approach a bit for this version to match some of the other libraries. Please let me know what you think.

[TheBrambleShark]

Changed up how I did things a bit by adding a new FixedWidthBuffer type. This type is intended only for serialization and allows for situations like this, where field order does not match the offset order:

[GenerateSerde]
public partial record class Person
(
    [property: FixedFieldInfo(0, 10)] string FirstName,
    [property: FixedFieldInfo(10, 10)] string LastName,
    [property: FixedFieldInfo(20, 5)] int EmployeeId,
    [property: FixedFieldInfo(28, 8, format: "MMddyyyy")] DateTime Birthday
)
{
    [FixedFieldInfo(25, 3, "D3")]
    [SerdeMemberOptions(SkipDeserialize = true)]
    public int Age => DateOnly.FromDateTime(DateTime.Today).Year - Birthday.Year;
}

See also: serdedotnet/serde#287