feat(wasm-encoder): add ComponentBuilder::instantiate_exports (#2655)

The core-level analogue `core_instantiate_exports` already exists on
`ComponentBuilder` and mirrors the `Instance::FromExports` variant of
the core instance section. The component-level side had no equivalent:
`ComponentInstanceSection::export_items` is public but only reachable
through the manual section-append path, not the builder facade.

Add `instantiate_exports` next to `instantiate` so callers rebuilding
a component structurally from a parsed one can round-trip
`ComponentInstance::FromExports` the same way core-level
`Instance::FromExports` already round-trips.

The signature follows the section-level `ComponentInstanceSection::
export_items` — `N: Into<ComponentExternName<'a>>` in the name slot,
preserving the rich name form (`implements` / `version_suffix` /
`external_id`) an item can carry. `&'a str` satisfies the bound via
the existing `From<&'a str> for ComponentExternName<'a>`, so callers
that only need a plain name write the same call as with
`core_instantiate_exports`.

Co-authored-by: Zachary Whitley <zachary.whitley@tegmentum.ai>

Author: zacharywhitley

crates/wasm-encoder/src/component/builder.rs

@@ -482,6 +482,19 @@ impl ComponentBuilder {
         base
     }
 
+    /// Creates a new component instance from the `exports` provided.
+    ///
+    /// Returns the index of the component instance created.
+    pub fn instantiate_exports<'a, E, N>(&mut self, debug_name: Option<&str>, exports: E) -> u32
+    where
+        E: IntoIterator<Item = (N, ComponentExportKind, u32)>,
+        E::IntoIter: ExactSizeIterator,
+        N: Into<ComponentExternName<'a>>,
+    {
+        self.component_instances().export_items(exports);
+        self.instances.add(debug_name)
+    }
+
     /// Declares a new `resource.drop` intrinsic.
     pub fn resource_drop(&mut self, ty: u32) -> u32 {
         self.canonical_functions().resource_drop(ty);