sqllogictest handles decimals inconsistently

[AdamGS]

As far as I can tell, decimal in the SLT suite are currently handled in the following way:

1. The two larger variants (128 and 256 bits) are handled as floats ('R' column type), and have their own formatter through the DecimalType type and BigDecimal that has set display accuracy (12 digits).
2. Smaller decimal types (32 and 64 bits) are handled as "unknown" ('?' column type) and use the fallback formatter.

This makes tests less accurate and causes inconsistencies between different code paths.

[AdamGS]

take