Fix npm install warnings

[twpayne]

Currently running npm install emits a couple of warnings:

$ npm install
npm WARN @fimbul/bifrost@0.21.0 requires a peer of tslint@^5.0.0 but none is installed. You must install peer dependencies yourself.
npm WARN tslint-consistent-codestyle@1.16.0 requires a peer of tslint@^5.0.0 but none is installed. You must install peer dependencies yourself.

added 2 packages from 4 contributors and audited 249476 packages in 5.588s

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found 0 vulnerabilities

It would be nice to fix these.