add String() method to Decimal

[veqryn]

Right now, Decimal only has a Float64 method and an unused toString(), which returns just the data type.

The Float64 method is great, but like all floats, it can introduce errors because of how floats work.

Example:

	// This outputs: 0.008216620000000001
	// It should be: 0.00821662
	fmt.Println((&duckdb.Decimal{
		Width: 18,
		Scale: 8,
		Value: big.NewInt(821662),
	}).Float64())

This PR adds a Stinger method allowing a lossless and correct conversion from Decimal to a string.

[taniabogatsch]

Hi @veqryn, thanks for your PR! I only had two nits - then it should be good to go. :)

[veqryn]

Thank you @taniabogatsch , should be good now

[taniabogatsch]

Thanks!