TypeScript Observers for codegen

[jgaehring]

These changes to the codegen repository corresponds to datafoodconsortium/connector-typescript#16, which contains the actual Connector library and distributables built with the Acceleo runner and the TypeScript compiler.

Note that I'm opening this on the 1.2.0 branch still, since main has yet to merge that release.

[jgaehring]

@lecoqlibre: I realized this only includes the TypeScript implementation of the observers. Do we want to wait until the Ruby and PHP implementations are ready?

I converted it back to a draft for the meantime. The variant changes and submodule work (#30) seemed to me like they ought to be merged first, because the TypeScript Observer still seems more susceptible to change until the Ruby/PHP work is done. And since alpha.11 of connector-typescript is already published on NPM, there's no urgency to merge this one into connector-codegen.