Improve Phlex compatibility

[allmarkedup]

Fixes a number issues with previewing Phlex components, as raised in #759

Nested render calls (i.e. within blocks passed to a component) should now work as expected:

def nested_rendering_example_scenario
  render Components::SimpleButton.new do
    render Components::SimpleIcon.new
  end
end

In addition, calls to Phlex helpers (such as plain) within blocks should also now work as expected:

def helper_example_scenario
  render Components::SimpleButton.new do
    plain "The button text"
  end
end

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