feat: Upgrade to support Gatsby 2 (finally 🎉)

[jessestuart]

What does this PR do?

Several fun things.

• Gatsby V2 support
• Lots of code cleanup & improvements — should resolve Error: Unable to find plugin "gatsby-source-s3-image" #121 and Multiple fixes (for Gatsby v2) #204.
• Migration from Flow => Typescript.
• Additional test coverage.

NB: this release is not backwards-compatible, due to changes in the Gatsby V2 API. If you'd like to continue using this source plugin with a Gatsby V1 site, pin to the v0.x package of this dependency. Otherwise, the primary change required for users is just in the shape of the graphql query — note the addition of the childrenFile field, e.g.,:

export const pageQuery = graphql`
  query($name: String) {
    allS3ImageAsset(filter: { EXIF: { DateCreatedISO: { eq: $name } } }) {
      edges {
        node {
          id
          EXIF {
            DateCreatedISO
            DateTimeOriginal
          }
          childrenFile {
            childImageSharp {
              thumbnailSizes: fluid(maxWidth: 512) {
                aspectRatio
                src
                srcSet
                sizes
              }
            }
          }
        }
      }
    }
  }

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I've verified the aforementioned changes in my personal site and haven't had any issues; in fact, performance / caching are significantly improved — builds with 100+ high-resolution JPEG's that used to cause Netlify builds to time out now no longer cause a problem. Of course, YMMV; don't hesitate to let me know if you see otherwise.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@04e428a). Click here to learn what that means.
The diff coverage is 72.72%.

@@            Coverage Diff            @@
##             master     #238   +/-   ##
=========================================
  Coverage          ?   72.72%           
=========================================
  Files             ?        2           
  Lines             ?       44           
  Branches          ?        6           
=========================================
  Hits              ?       32           
  Misses            ?       12           
  Partials          ?        0

Impacted Files	Coverage Δ
src/utils.ts	100% <100%> (ø)
src/source-nodes.ts	62.5% <62.5%> (ø)

---

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[jessestuart]

🎉 This PR is included in version 1.0.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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