Add aliases to JsonValue to enum value to be decoded from different JSON values

[mateusfccp]

Solves #1380.

Adds a parameter to JsonValue called aliases, which accepts a list of values that can be alternatively used to decode a JSON.

Example:

enum StatusCode {
  @JsonValue(200, aliases: [201, 202])
  success,
  @JsonValue(301)
  movedPermanently,
  @JsonValue(302)
  found,
  @JsonValue(500)
  internalServerError,
}

With the code above, all of 200, 201 and 202 would be decoded as StatusCode.success.

When calling toJson, however, the value parameter is used. In this case, 200.

Notes

• The $enumDecode and $enumDecodeNullable were not changed because they are public and there are packages that depend on them. Instead, $enumDecodeWithDecodeMap and $enumDecodeNullableWithDecodeMap were introduced. We may want to deprecate the former at some point and remove them in a major version.

[kevmoo]

Ideally we'd have a couple of tests to validate the failure cases, but I get that those can be tricky to add.

[mateusfccp]

@kevmoo What would be the failure cases for this PR? Can you give an example?

[kevmoo]

@kevmoo What would be the failure cases for this PR? Can you give an example?

Unsupported types in the set is the big one.
null isn't supported, right?
Those types of things.

[mateusfccp]

@kevmoo What would be the failure cases for this PR? Can you give an example?

Unsupported types in the set is the big one. null isn't supported, right? Those types of things.

Ah, yes, the idea is to mimic value, so until we have proper union types in Dart, we use Object? but should only accept String and int.

I am going to work on this test.

[kevmoo]

You'll need to rebase this fix.

[mateusfccp]

You'll need to rebase this fix.

I rebased to master and included the requested test.

In my local machine, a test was failing, but I didn't change anything related to it (the *ToJson generator).

[mateusfccp]

Mmm, I had made a regular merge, but GitHub is saying to me that "this branch must not contain merge commits", so I rewrote the commits using a rebase instead, but it is still giving me this message.

[mateusfccp]

Well, the test that did not pass locally passed here, so we are good with it.