Auto detect entry points

[hypebright]

We could add an auto_detect argument to build_js, that will auto discover the entry points in a given folder (or in the default srcjs). Currently when you're doing component based bundling you constantly need to provide all the entry points, like so:

build_js(entry_points = c("componentA.js", "componentB.js", "componentC.js"))

and if you accidentally forget to give the entry_point arg, you end up with a main.js file again 🙃.

It could look like this instead:

build_js(auto_detect = TRUE)

Default would be FALSE (or TRUE even, if that works in combination while ensuring backwards compatibility?) and it would automatically look for .js files in the directory given in dir.