Base auto-capture logic on yield calls in template source

[kaspth]

Extracted from #44

Currently, we're basing our auto-capture logic on whether there's a
single argument passed to render, e.g.:

render "card" do |partial|
  partial.yield :title, t(".title")
end

But if users want to pass extra content via an explicit <%= yield something %>,
we don't support that.

This changes our logic to look for <%= yield something %> calls in the
template source and not auto-capture in that case. We will pass the partial
on as an extra argument in case callers want to expose that as a named block
argument.

We'll still auto-capture in case a template has non-capturing yields like
<%= yield :title %>.

[kaspth]

Note: this also relies on some code structure in ActionView::PartialRenderer that was added in 6.1. I don't know how tied we are to backwards compatibility, but with the _layout_for override users would just need to insert a plain <%= yield %> into their partials, or we could look at explicit versions support for Rails 5.2 and possibly further down. What do you say @andrewculver?

[andrewculver]

@kaspth No need to provide backward compatibility. Happy to support 6.1 and up.

[domchristie]

Just noticing the %>: does this work with HAML?

[kaspth]

Originally I wanted to define this after template compilation, so we'd be Template Handler independent, but I just realized we'd need to trigger compilation for that to work (and that's why it didn't work).

I've never used anything besides ERB, how does HAML syntax go by the way? Like this?

%= yield
%= yield :title

Do you know what Slim looks like?

[domchristie]

I'm not a HAML/Slim user, but the %> got me thinking.

I've never used anything besides ERB, how does HAML syntax go by the way? Like this?

Yep I think so.

Slim looks similar http://slim-lang.com/