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
```