Add Component::Instance#get_export_index/#get_resource and WIT resour…

[teleyos]

…ce support

Instance previously only exposed #get_func, making it impossible to fully drive a component that exports a WIT resource (e.g. a constructor plus instance methods) from Ruby: there was no way to look up the resource's exported type, and Val::Resource/Type::Own/Type::Borrow conversions were hard not_implemented! stubs.

• Instance#get_export_index / #get_resource wrap wasmtime's own methods of the same name; the shared lookup helper now accepts a previously-resolved ExportIndex (bare or nested in an Array) so lookups can be cached and chained, matching wasmtime's own doc example, and internally prefers get_export_index over get_export since the ComponentItem it discards is never used.
• New Wasmtime::Component::Resource wraps ResourceAny + its owning Store, requiring an explicit #resource_drop (wasmtime requires every ResourceAny, even borrows, to be explicitly dropped; there's no safe GC finalizer hook for re-entering a Store during collection).
• New Wasmtime::Component::ExportIndex and ResourceType wrap the matching wasmtime types; both are inert Copy data with no Store dependency, so unlike Resource they need no GC mark.
• convert.rs now lifts/lowers resource values for the Func::invoke path (calling exported functions), and guards against passing a Resource obtained from one Store into a call on a different Store, which would otherwise silently index into the wrong store's resource table.
• Host-defined import functions (Linker#root.func_new) still don't support resources in either direction; that path never had a Ruby-visible Store handle to thread through, and stays out of scope here.

[jeffcharles]

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