Rename "contentTypes" API to "content"

[themeblvd]

Currently, one of the key features, and base principles, of the framework is to add types of content that can be managed by the end-user.

The current API for this is named contentTypes. For example:

const app = backroad();

app.contentTypes.add({...});

I believe this is a bit limiting for future grown of that particular segment of the API (i.e. around configuring content). I would prefer to see it named something more general and simple like content.

However, with that said adding a content "type" is still the base of everything. So I think it would still be acceptable that INSTEAD of this:

app.content.addType({...});

We end up something like this:

app.content.add({...});

Having the method add() is arguably too implicitly named; however, I believe it's easiest to document to developers and it doesn't need to be explicit because this is all formed around it being the "content type API".

Later, things that aren't specificlly a content type could theoretically be named more explicitly like:

app.content.addGroup({...}); // Maybe a method for creating taxonomies?