Use RUBY_TYPED_EMBEDDABLE in BigDecimal

[mrkn]

Ruby 3.3 introduces RUBY_TYPED_EMBEDDABLE. Now a BigDecimal with a small amount of digits can embed its internal data.

[mrkn]

Here’s a recently published article that provides helpful information on this topic.
https://railsatscale.com/2025-06-03-implementing-embedded-typeddata-objects/

[byroot]

Implemented in #507