feat: Add render tag

The main functionality missing is `as <name>` should be optional,
defaulting to the partial's file name.

For background, see
- https://shopify.dev/docs/api/liquid/tags/render
- https://github.com/Shopify/liquid/blob/master/lib/liquid/tags/render.rb
- https://github.com/Shopify/liquid/blob/master/test/integration/tags/render_tag_test.rb

Fixes cobalt-org#471

crates/core/src/runtime/stack.rs

@@ -91,12 +91,14 @@ impl<P: super::Runtime, O: ObjectView> super::Runtime for StackFrame<P, O> {
     }
 }
 
-pub(crate) struct GlobalFrame<P> {
+/// A stack frame that only provides a sandboxed set of globals
+pub struct GlobalFrame<P> {
     parent: P,
     data: std::cell::RefCell<Object>,
 }
 
 impl<P: super::Runtime> GlobalFrame<P> {
+    /// Override globals for `parent`
     pub fn new(parent: P) -> Self {
         Self {
             parent,

crates/lib/src/stdlib/tags/mod.rs

@@ -3,6 +3,7 @@ mod cycle_tag;
 mod include_tag;
 mod increment_tags;
 mod interrupt_tags;
+mod render_tag;
 
 pub use self::assign_tag::AssignTag;
 pub use self::cycle_tag::CycleTag;
@@ -11,3 +12,4 @@ pub use self::increment_tags::DecrementTag;
 pub use self::increment_tags::IncrementTag;
 pub use self::interrupt_tags::BreakTag;
 pub use self::interrupt_tags::ContinueTag;
+pub use self::render_tag::RenderTag;