Fixes 2 warnings from npm

[akash1810]

What does this change?

Recently ran ./script/setup and saw a few warnings (see below). This fixes the prepublish deprecation warning and the missing repository field warning. There's a third warning about a missing license field, but I'm not sure what license we need.

I vaguely remember us having issues with prepare in the past, however I think that when we were installing directly from GitHub rather than NPM, so this should be ok.

Original terminal output

➜ ./script/setup
npm WARN prepublish-on-install As of npm@5, `prepublish` scripts are deprecated.
npm WARN prepublish-on-install Use `prepare` for build steps and `prepublishOnly` for upload-only.
npm WARN prepublish-on-install See the deprecation note in `npm help scripts` for more information.

> @guardian/[email protected] prepublish /Users/akash_askoolum/code/cdk
> tsc

npm WARN @guardian/[email protected] No repository field.
npm WARN @guardian/[email protected] No license field.

added 61 packages from 29 contributors, removed 17 packages, updated 26 packages and audited 1159 packages in 12.345s

98 packages are looking for funding
  run `npm fund` for details

found 0 vulnerabilities

Does this change require changes to existing projects or CDK CLI?

No.

How to test

n/a

How can we measure success?

Fewer warnings from NPM.

Have we considered potential risks?

Prepare might run at a different lifecycle point, however the docs suggests prepublish is:

prepublish (DEPRECATED)
Same as prepare

🤞🏽

[jamie-lynch]

This gives a bit more context into the name change: https://docs.npmjs.com/cli/v6/using-npm/scripts#prepare-and-prepublish