V14: Build and release the type declarations of the Backoffice to npm

[iOvergaard]

Description

Fixes #16719

The CMS ships with a bundled, non-declarative version of the Backoffice, but for NPM, we need to ship the type declarations. This PR adds a parallel job that aims to build the npm-version of the Backoffice before the myget/npm stages are required.

Other benefits

• Separated the Backoffice install into its own template to re-use the same npm cache/install/version in 3 places
• Now calculating the correct version also for the Backoffice shipped with the CMS (one of the API calls would give a static version before)
• Shaved off almost 1 minute of the Build stage

The new job finishes in 3m 26s usually, which is long before the "Build Umbraco CMS" job finishes, so it doesn't add to the total build time:

[Zeegaan]

LGTM 👍