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.c

@@ -7,6 +7,7 @@
 #include "bh_common.h"
 #include "bh_assert.h"
 #include "bh_log.h"
+#include "wasm_native.h"
 #include "wasm_runtime_common.h"
 #include "wasm_memory.h"
 #if WASM_ENABLE_INTERP != 0
@@ -5331,3 +5332,24 @@ wasm_runtime_get_version(uint32_t *major, uint32_t *minor, uint32_t *patch)
     *minor = WAMR_VERSION_MINOR;
     *patch = WAMR_VERSION_PATCH;
 }
+
+bool
+wasm_runtime_is_import_func_linked(const char *module_name,
+                                   const char *func_name)
+{
+    return wasm_native_resolve_symbol(module_name, func_name, NULL, NULL, NULL,
+                                      NULL);
+}
+
+bool
+wasm_runtime_is_import_global_linked(const char *module_name,
+                                     const char *global_name)
+{
+#if WASM_ENABLE_LIBC_BUILTIN != 0
+    WASMGlobalImport global = { 0 };
+    return wasm_native_lookup_libc_builtin_global(module_name, global_name,
+                                                  &global);
+#else
+    return false;
+#endif
+}