Make the frontend a module for customview

[syucream]

Current custom view is being in chaos ...

• No official API, so some developers will call various exported variables/functions directly. So custom views will be broken easily with this repo's changes.
• No node module management for custom views. Mostly it requires to install additional modules in "this" repo.
• It should require filesystem hack to install custom views, e.g. put their custom views in frontend/src/customview/..., a non-scalable way ...

So I wanna propose a smarter integration way between core (this repo) and custom views. Just providing this frontend implementations as a node module, then make developers to extend it theirselves. Thats like class-extension in some object-oriented languages, class MyCustomView extends PagodaCore { ....
Its an example customview project to extend that with a simple React components.
https://github.com/syucream/pagoda-customview-example

[syucream]

Separated a large, independent improvement part as another PR:
#1318

[syucream]

Now it works fine with my super simple sample project! https://github.com/syucream/pagoda-customview-example

[syucream]

And also I've added a workflow to auto-publish the pagoda-core npm module, triggered on a new release tag named pagoda-core-x.y.z.