fix: add decimal lib in dependencies

Currently, `decimal` is added indirectly because `ecto` is an (optional)
dependency in `mix.exs`.

However, if the project where you want to add `logger_json` doesn't have `ecto`
in its dependencies, `decimal` is not added and then `logger_json` doesn't
compile:

```
==> logger_json
Compiling 17 files (.ex)
    error: Decimal.__struct__/0 is undefined, cannot expand struct Decimal. Make sure the struct name is correct. If the struct name exists and is correct but it still cannot be found, you likely have cyclic module usage in your code
    │
 39 │   def encode(%Decimal{} = decimal, _redactors), do: decimal
    │              ^
    │
    └─ lib/logger_json/formatter/redactor_encoder.ex:39:14: LoggerJSON.Formatter.RedactorEncoder.encode/2

== Compilation error in file lib/logger_json/formatter/redactor_encoder.ex ==
** (CompileError) lib/logger_json/formatter/redactor_encoder.ex: cannot compile module LoggerJSON.Formatter.RedactorEncoder (errors have been logged)
    lib/logger_json/formatter/redactor_encoder.ex:39: (module)
could not compile dependency :logger_json, "mix compile" failed. Errors may have been logged above. You can recompile this dependency with "mix deps.compile logger_json --force", update it with "mix deps.update logger_json" or clean it with "mix deps.clean logger_json"
```

You can easily reproduce the bug by removing the `ecto` dependency in the
`logger_json` `mix.exs` file. You will see that the `decimal` dependency
disappears in `deps.tree` and that the lib doesn't compile anymore
(same error as above).

```
➜  mix deps.get
Resolving Hex dependencies...
Resolution completed in 0.055s
New:
  castore 1.0.8
  dialyxir 1.4.3
  earmark_parser 1.4.41
  erlex 0.2.7
  ex_doc 0.34.2
  excoveralls 0.18.3
  jason 1.4.4
  junit_formatter 3.4.0
  makeup 1.1.2
  makeup_elixir 0.16.2
  makeup_erlang 1.0.1
  mime 2.0.6
  nimble_parsec 1.4.0
  plug 1.16.1
  plug_crypto 2.1.0
  stream_data 1.1.1
  telemetry 1.3.0
All dependencies are up to date
```

It may be interesting to refactor the code in another PR so that `decimal`
encoding is not compiled when `decimal` dependency doesn't exist in the project.

mix.exs

@@ -36,6 +36,7 @@ defmodule LoggerJSON.Mixfile do
   defp deps do
     [
       {:jason, "~> 1.4"},
+      {:decimal, "~> 2.0"},
       {:plug, "~> 1.15", optional: true},
       {:ecto, "~> 3.11", optional: true},
       {:telemetry, "~> 1.0", optional: true},