Adds mergeable state property in the Github PR struct

[Juanpe]

This PR adds new property in Github PR struct. This property is mergeable. It indicates whether or not the pull request can be merged based on the existence of merge conflicts.

You can find more details in Github API documentation:

• Pull Request
  • mergeable

The motivation is that it could be useful to know if the PR is mergeable or not. For example to add a label like merge conflicts

[danger-public]

	Fails
🚫	danger-swift` failed.

Log

�[31mERROR: Failed to parse JSON: typeMismatch(Swift.String, Swift.DecodingError.Context(codingPath: [CodingKeys(stringValue: "danger", intValue: nil), CodingKeys(stringValue: "github", intValue: nil), CodingKeys(stringValue: "pr", intValue: nil), CodingKeys(stringValue: "mergeable", intValue: nil)], debugDescription: "Expected to decode String but found a number instead.", underlyingError: nil))
�[0;0m�[31mERROR: Dangerfile eval failed at Dangerfile.swift
�[0;0m�[31mERROR: Could not get the results JSON file at /var/folders/17/5mc7816d3mndxjqgplq6057w0000gn/T/danger-response.json
�[0;0m

Generated by 🚫 dangerJS against d7b5813

[f-meloni]

I think mergeable is an optional Bool on the API that danger uses.
You can verify this by running danger-swift pr https://github.com/danger/swift/pull/318 --json > file.json this will generate the json that Danger decodes on file.json

[417-72KI]

MergeableState is an object defined in GraphQL API, which seems not to be not used on danger.
Danger uses REST API, and it says

The value of the mergeable attribute can be true, false, or null. If the value is null, then GitHub has started a background job to compute the mergeability. After giving the job time to complete, resubmit the request. When the job finishes, you will see a non-null value for the mergeable attribute in the response. If mergeable is true, then merge_commit_sha will be the SHA of the test merge commit.

(ref: https://docs.github.com/en/rest/reference/pulls#get-a-pull-request)