Expose language field in GraphQL blob type

What does this MR do and why?

Contributes to #348145 (closed)

Return the language of the blob for syntax highlighter.

Screenshots or screen recordings

How to set up and validate locally

http://127.0.0.1:3000/-/graphql-explorer

{
  project(fullPath: "flightjs/Flight") {
    repository {
      blobs(paths: ".travis.yml") {
        nodes {
          name
          language
        }
      }
    }
  }
}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.