feat(module): allow adding module to Nuxt config within layer

[DamianGlowala]

resolves #361

This allows for adding a module entry to a Nuxt config file within a layer using --layer <layer_name>.

[pi0]

Without considering layer names, isn’t it same as —cwd ?

[DamianGlowala]

I think setting --cwd pointing to the layer dir would affect other places as well, e.g. where module add looks for package.json or where it adds a dependency using nypm util. Correct me if I'm wrong though.

(There might be a better way to achieve it as well, let me know if you have any idea.)

[pi0]

I think what is expected is that the module is added to the closest package.json and nuxt.config files found to the specified cwd. (ie: We need a look-up search for both). nypm should do this via package managers by default.

I think it would probably be best first action to resolve any issues that prevents this.

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In next steps, —layer would be also good idea but i expect that it loads nuxt.config, and tries to match a layer by its name (which used in extends key) basically as a convenient shortcut and also it should implement safety check that we don’t write to readonly (remote or dependency) layers but instead ask to directly add to the root.