Introduce kubevirt/{package.json|yarn.lock}

[mareklibra]

Kubevirt npm dependencies are independent from openshift's frontend/package.json and frontend/yarn.lock.

With this patch, we can use yarn.lock feature even for kubevirt-specific
dependencies without risk of mergeconflict with openshift package.json/yarn.lock.

[mareklibra]

Thanks @vojtechszocs for this idea!

[mareklibra]

Linter is failing to resolve kubevirt/node_modules packages when using webpack's
resolver to handle two node_modules directories.

This can be fixed i.e. via https://www.npmjs.com/package/eslint-import-resolver-alias
but it would require modification of
frontend/{package.json|yarn.lock}, frontend/eslintrc and frontend/webpack.config.ts.

So it would still not address our main concern - eliminate conflicts with OKD sources.

Leads to following changes to this PR:

• frontend/{package.json|yarn.lock} dependencies and frontend/webpack.config.ts stay untouched
• new frontend/package.json preinstall manages kubevirt/{package.json|yarn.lock}
• new frontend/package.json postinstall adds all kubevirt/node_modules to frontend/node_modules

With this approach:

• all manipulations with dependencies stay clear at the first glance, as all changes are
  in the preinstall and postinstall scripts only
• still minimum changes to okd code
  • just the one-liner for package.json scripts
  • no additional packages needed for the frontend/package.json
  • no change to webpack.config.ts

[rawagner]

we could create .gitignore in kubevirt dir

[rawagner]

LGTM