Added basic support for TypeScript

[oliverguenther]

This supersedes #3735 with fixed npm-shrinkwrap.json.

/cc @furinvader, @romanroe: You should clarifiy the role of typings/tsd.d.ts, since its specified in the config, but excluded from Git.

Also, please add some kind of documentation before merging this.

[TeatroIO]

I've prepared a stage to preview changes. Open stage or view logs.

[oliverguenther]

https://community.openproject.org/work_packages/21913/activity

[ulferts]

Copying my comment from #3735:

However, instead of "npm install" I had to do
npm install --save typescript
npm install --save ts-loader

Would that also be necessary for users wanting to install OpenProject after this is merged? We would then have to update our testing and delivery pipeline.

[oliverguenther]

No, --save is only required to introduce the dependency into package.json. npm i(nstall) is sufficient on the user side of things.

[romanroe]

The file typings/tsd.d.ts gets generated with tsd install and is required for the build. The content is based on tsd.json.

I am a bit lost between https://community.openproject.org/work_packages/21913/activity and this pull request. What can I do to help with the TypeScript integration?