Allowing templates to contain inherited blocks/sections (à la Twig, Blade, et al.)

[Patrick Nelson]

RFC here: https://github.com/silverstripe/silverstripe-framework/issues/7498

Opening this up for a little more discussion and exposure. Is there support for this sort of functionality? Please read through Twig documentation (here: https://twig.symfony.com/doc/2.x/tags/extends.html)  for an example of what I'm after for SilverStripe's native template language. This would allow for more advanced mixing of HTML that is currently not inherently possible simply by using includes or using our one variable (i.e. $Layout) two level layout system.

Let me know what you guys think! Please offer critiques, alternatives, syntax changes and anything else that comes to mind either here or (preferably) on the issue itself. Thanks!