Improve wasm-c-api instantiation-time linking (#1902)

Add APIs to help prepare the imports for the wasm-c-api `wasm_instance_new`:
- wasm_importtype_is_linked
- wasm_runtime_is_import_func_linked
- wasm_runtime_is_import_global_linked
- wasm_extern_new_empty

For wasm-c-api, developer may use `wasm_module_imports` to get the import
types info, check whether an import func/global is linked with the above API,
and ignore the linking of an import func/global with `wasm_extern_new_empty`.

Sample `wasm-c-api-import` is added and document is updated.

core/iwasm/common/wasm_runtime_common.h

@@ -993,6 +993,14 @@ void
 wasm_runtime_destroy_custom_sections(WASMCustomSection *section_list);
 #endif
 
+WASM_RUNTIME_API_EXTERN bool
+wasm_runtime_is_import_func_linked(const char *module_name,
+                                   const char *func_name);
+
+WASM_RUNTIME_API_EXTERN bool
+wasm_runtime_is_import_global_linked(const char *module_name,
+                                     const char *global_name);
+
 #ifdef __cplusplus
 }
 #endif