refactor: rename wrenGetArgument* to wrenGetSlot* and add IS_LIST macro for slot-based API

Migrate the foreign method argument access API from an argument-oriented naming
scheme to a slot-based one, renaming wrenGetArgumentCount to wrenGetSlotCount,
wrenGetArgumentBool to wrenGetSlotBool, and all other argument accessors
accordingly. Update every call site across the VM, modules (io, timer, meta,
random), and test files (benchmark, foreign_class) to use the new names. The
slot getters now assert the correct type at runtime rather than silently
returning defaults, shifting safety responsibility to the caller for
performance. Also add the IS_LIST type-checking macro to wren_value.h alongside
the existing IS_* macros.

src/include/wren.h

@@ -120,7 +120,7 @@ typedef struct
   //
   // If this is `NULL`, Wren discards any printed text.
   WrenWriteFn writeFn;
-  
+
   // The number of bytes Wren will allocate before triggering the first garbage
   // collection.
   //
@@ -237,8 +237,8 @@ void wrenReleaseValue(WrenVM* vm, WrenValue* value);
 // the foreign object's data.
 void* wrenAllocateForeign(WrenVM* vm, size_t size);
 
-// Returns the number of arguments available to the current foreign method.
-int wrenGetArgumentCount(WrenVM* vm);
+// Returns the number of slots available to the current foreign method.
+int wrenGetSlotCount(WrenVM* vm);
 
 // The following functions read one of the arguments passed to a foreign call.
 // They may only be called while within a function provided to
@@ -260,32 +260,37 @@ int wrenGetArgumentCount(WrenVM* vm);
 //
 // It is an error to pass an invalid argument index.
 
-// Reads a boolean argument for a foreign call. Returns false if the argument
-// is not a boolean.
-bool wrenGetArgumentBool(WrenVM* vm, int index);
+// Reads a boolean value from [slot].
+//
+// It is an error to call this if the slot does not contain a boolean value.
+bool wrenGetSlotBool(WrenVM* vm, int slot);
 
-// Reads a numeric argument for a foreign call. Returns 0 if the argument is not
-// a number.
-double wrenGetArgumentDouble(WrenVM* vm, int index);
+// Reads a number from [slot].
+//
+// It is an error to call this if the slot does not contain a number.
+double wrenGetSlotDouble(WrenVM* vm, int slot);
 
-// Reads a foreign object argument for a foreign call and returns a pointer to
-// the foreign data stored with it. Returns NULL if the argument is not a
-// foreign object.
-void* wrenGetArgumentForeign(WrenVM* vm, int index);
+// Reads a foreign object from [slot] and returns a pointer to the foreign data
+// stored with it.
+//
+// It is an error to call this if the slot does not contain an instance of a
+// foreign class.
+void* wrenGetSlotForeign(WrenVM* vm, int slot);
 
-// Reads an string argument for a foreign call. Returns NULL if the argument is
-// not a string.
+// Reads a string from [slot].
 //
 // The memory for the returned string is owned by Wren. You can inspect it
-// while in your foreign function, but cannot keep a pointer to it after the
+// while in your foreign method, but cannot keep a pointer to it after the
 // function returns, since the garbage collector may reclaim it.
-const char* wrenGetArgumentString(WrenVM* vm, int index);
+//
+// It is an error to call this if the slot does not contain a string.
+const char* wrenGetSlotString(WrenVM* vm, int slot);
 
-// Creates a handle for the value passed as an argument to a foreign call.
+// Creates a handle for the value stored in [slot].
 //
 // This will prevent the object that is referred to from being garbage collected
 // until the handle is released by calling [wrenReleaseValue()].
-WrenValue* wrenGetArgumentValue(WrenVM* vm, int index);
+WrenValue* wrenGetSlotValue(WrenVM* vm, int slot);
 
 // The following functions provide the return value for a foreign method back
 // to Wren. Like above, they may only be called during a foreign call invoked