Squash on green support

[ashfurrow]

This PR does a few things:

• Update our TypeScript version so we can use as const (also supported on our Peril install).
• Adds squash-on-green support.
• Adds unit test.

I've tried to structure the code so that adding a rebase-on-green should be straightforward. Let me know what I can clarify!

Note: once merged, this change will be immediately live for our Peril install. If something goes wrong, the logs are here 🔐 and reverting the PR merge would make things work again.

[ashfurrow]

Okay let's test this out ...

[ashfurrow]

Hmm, the label-adding code worked but I'm seeing problems with the mergeOnGreen.ts file:

2020-09-11T13:53:21.996491+00:00 app[web.1]: Fri, 11 Sep 2020 13:53:21 GMT runFromSameHost Unable to evaluate the Dangerfile
2020-09-11T13:53:21.996506+00:00 app[web.1]:  peril-downloaded-artsy/peril-settings@org/mergeOnGreen.ts:30
2020-09-11T13:53:21.996508+00:00 app[web.1]:         let commitTitle = mergeLabel.commitGenerator ? .(number)
2020-09-11T13:53:21.996508+00:00 app[web.1]:                                                        ^
2020-09-11T13:53:21.996508+00:00 app[web.1]: 
2020-09-11T13:53:21.996509+00:00 app[web.1]: SyntaxError: Unexpected token .
2020-09-11T13:53:21.996510+00:00 app[web.1]:     at Module._compile (internal/modules/cjs/loader.js:723:23)
2020-09-11T13:53:21.996511+00:00 app[web.1]:     at Object.requireFromString [as default] (/app/node_modules/require-from-string/index.js:28:4)
2020-09-11T13:53:21.996511+00:00 app[web.1]:     at Object.<anonymous> (/app/node_modules/danger/distribution/runner/runners/inline.js:144:63)
2020-09-11T13:53:21.996513+00:00 app[web.1]:     at step (/app/node_modules/danger/distribution/runner/runners/inline.js:32:23)
2020-09-11T13:53:21.996513+00:00 app[web.1]:     at Object.next (/app/node_modules/danger/distribution/runner/runners/inline.js:13:53)
2020-09-11T13:53:21.996513+00:00 app[web.1]:     at /app/node_modules/danger/distribution/runner/runners/inline.js:7:71
2020-09-11T13:53:21.996514+00:00 app[web.1]:     at new Promise (<anonymous>)
2020-09-11T13:53:21.996514+00:00 app[web.1]:     at __awaiter (/app/node_modules/danger/distribution/runner/runners/inline.js:3:12)
2020-09-11T13:53:21.996515+00:00 app[web.1]:     at Object.exports.runDangerfileEnvironment (/app/node_modules/danger/distribution/runner/runners/inline.js:105:136)
2020-09-11T13:53:21.996516+00:00 app[web.1]:     at runDangerEvent (/app/api/out/runner/run.js:101:28)
2020-09-11T13:53:21.996516+00:00 app[web.1]:     at process._tickCallback (internal/process/next_tick.js:68:7)

I'm going to revert this for now since this will break merge-on-green for all our repos. I'll follow-up with another PR that uses less fancy syntax, since it's apparently causing issues with Peril.