Proposal: Should we include a matcher?

I had a fail based on two long strings which I fixed this way:


After putting a `console.log` in the dangerfile. This ain't good.

---

Code looked like this:

```ts
import dtsGenerator from "./scripts/danger-dts"
const currentDTS = dtsGenerator()
const savedDTS = fs.readFileSync("source/danger.d.ts").toString()
if (currentDTS !== savedDTS) {
const message = "There are changes to the Danger DSL which are not reflected in the current danger.d.ts."
const idea = "Please run yarn declarations and update this PR."
fail(`${message}
${idea}`)
}
```

We already have `jest-diff` inside the app, due to `jest-config`, so what about instead we provide something like:

```ts
import dtsGenerator from "./scripts/danger-dts"
const currentDTS = dtsGenerator()
const savedDTS = fs.readFileSync("source/danger.d.ts").toString()

const message = "There are changes to the Danger DSL which are not reflected in the current danger.d.ts."
const idea = "Please run yarn declarations and update this PR."
const failureMessage =`${message}
${idea}`

danger.utils.checkEqual(fail, failureMessage, currentDTS, savedDTS)
```

Where we can present jest-matcher like messages inline. For example:


or


Start by locating the existing danger.utils entry point and reviewing scripts/danger-dts and source/danger.d.ts, which are used in the example. Check how the existing jest-diff dependency is available, then define and test the proposed checkEqual API and its inline diff output.

Written by the indexing model from the issue text.