Style: Partially apply clang-tidy's cppcoreguidelines-pro-type-member-init

Didn't commit all the changes where it wants to initialize a struct
with `{}`. Should be reviewed in a separate PR.

Option `IgnoreArrays` enabled for now to be conservative, can be
disabled to see if it proposes more useful changes.

Also fixed manually a handful of other missing initializations / moved
some from constructors.

modules/visual_script/visual_script_builtin_funcs.cpp

@@ -1180,8 +1180,8 @@ void VisualScriptBuiltinFunc::exec_func(BuiltinFunc p_func, const Variant **p_in
 
 class VisualScriptNodeInstanceBuiltinFunc : public VisualScriptNodeInstance {
 public:
-	VisualScriptBuiltinFunc *node;
-	VisualScriptInstance *instance;
+	VisualScriptBuiltinFunc *node = nullptr;
+	VisualScriptInstance *instance = nullptr;
 
 	VisualScriptBuiltinFunc::BuiltinFunc func;