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.

visual_script_expression.cpp

@@ -1299,8 +1299,8 @@ bool VisualScriptExpression::_compile_expression() {
 
 class VisualScriptNodeInstanceExpression : public VisualScriptNodeInstance {
 public:
-	VisualScriptInstance *instance;
-	VisualScriptExpression *expression;
+	VisualScriptInstance *instance = nullptr;
+	VisualScriptExpression *expression = nullptr;
 
 	//virtual int get_working_memory_size() const override { return 0; }
 	//execute by parsing the tree directly