Fix warnings by conditionally compiling Decimal support

[fhunleth]

This fixes the following Elixir 1.17 warnings when :decimal isn't
included in the dependency list:

==> jason
    warning: Decimal.new/1 is undefined (module Decimal is not available or is yet to be defined)
    │
 94 │         decimal.new(string)
    │                 ~
    │
    └─ (jason 1.4.3) lib/decoder.ex:94:17: Jason.Decoder.float_decode_function/1

warning: struct Decimal.Error is undefined (module Decimal.Error is not available or is yet to be defined)
└─ (jason 1.4.3) lib/decoder.ex: Jason.Decoder.float_decode_function/1

     warning: Decimal.to_string/2 is undefined (module Decimal is not available or is yet to be defined)
     │
 242 │     [?", decimal.to_string(value, :normal), ?"]
     │                  ~
     │
     └─ (jason 1.4.3) lib/encode.ex:242:18: Jason.Encode.struct/4

     warning: Decimal.to_string/1 is undefined (module Decimal is not available or is yet to be defined)
     │
 231 │     [?", decimal.to_string(value), ?"]
     │                  ~
     │
     └─ (jason 1.4.3) lib/encoder.ex:231:18: Jason.Encoder.Decimal.encode/2

[michalmuskala]

I'm happy to merge this, thank you for working on this. That said, I think with some small changes we can make this even better

[michalmuskala]

I think at this point, we should just remove the old work-around for silencing the warnings

Suggested change

	if Code.ensure_loaded?(Decimal) do
	defp float_decode_function(%{floats: :decimals}) do
	fn string, token, skip ->
	# silence xref warning
	decimal = Decimal
	try do
	decimal.new(string)
	rescue
	Decimal.Error ->
	token_error(token, skip)
	end
	if Code.ensure_loaded?(Decimal) do
	defp float_decode_function(%{floats: :decimals}) do
	fn string, token, skip ->
	try do
	Decimal.new(string)
	rescue
	Decimal.Error ->
	token_error(token, skip)
	end

[michalmuskala]

Additionally, do you think it would make sense to have a function that would emit a nice error if this option is passed but decimal was not included? I think this would be a good usability upgrade.

[fhunleth]

I think at this point, we should just remove the old work-around for silencing the warnings

Oh good. I almost did that and then wasn't 100% sure there wasn't some weird edge case. I added a commit to the PR that removes them.

[fhunleth]

Additionally, do you think it would make sense to have a function that would emit a nice error if this option is passed but decimal was not included? I think this would be a good usability upgrade.

Yes, that seems nice. I raised in the decoder with a message I think is consistent with the library. The encoder calls seemed fine since they required Decimal structs to be passed.

[michalmuskala]

Thank you! ❤️

[michalmuskala]

This was released in 1.4.4 today. Sorry for the delay