chore: migrate the code to typescript

[DominicKramer]

BREAKING CHANGE: As a result of the migration, the way in which this package is required and used has changed slightly.

That is, the package is required and a LoggingBuyan object is created as follows on Node 6+. In particular, note the {} braces used in the require statement and the use of new which is now mandatory.

const {LoggingBunyan} = require('@google-cloud/logging-bunyan');

// Creates a Bunyan Stackdriver Logging client
const loggingBunyan = new LoggingBunyan();

On Node 4, the require syntax is slightly different, but the use of new is still required.

const LoggingBunyan = require('@google-cloud/logging-bunyan').LoggingBunyan;

// Creates a Bunyan Stackdriver Logging client
const loggingBunyan = new LoggingBunyan();

[googlebot]

So there's good news and bad news.

👍 The good news is that everyone that needs to sign a CLA (the pull request submitter and all commit authors) have done so. Everything is all good there.

😕 The bad news is that it appears that one or more commits were authored by someone other than the pull request submitter. We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that here in the pull request.

Note to project maintainer: This is a terminal state, meaning the cla/google commit status will not change from this State. It's up to you to confirm consent of the commit author(s) and merge this pull request when appropriate.

[ofrobots]

LGTM. Note that I edited the issue title to fix capitalization. See https://github.com/angular/angular.js/blob/master/DEVELOPERS.md#subject