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.

core/core_constants.cpp

@@ -41,7 +41,7 @@ struct _CoreConstant {
 	StringName enum_name;
 	bool ignore_value_in_docs = false;
 #endif
-	const char *name;
+	const char *name = nullptr;
 	int value = 0;
 
 	_CoreConstant() {}