Add three new public C API functions to the Wren embedding API that enable
foreign methods to create and populate lists. wrenEnsureSlots grows the
foreign slot stack to guarantee space for temporary work, wrenSetSlotNewList
stores a new empty list in a slot, and wrenInsertInList inserts a value from
one slot into a list at another slot at a given index (supporting negative
indices for relative positioning). The implementation includes a new internal
ensureStack helper that reallocates the fiber's stack and updates all
pointers (stack top, call frame starts, open upvalues, and foreign stack
start) when the reallocation moves the stack. New test files (lists.c,
lists.h, lists.wren) exercise creation, appending, insertion, and negative
index insertion, and the existing slots test is extended with an ensure test
that verifies slot count growth and slot usability.
The old wrenReturn___() functions only wrote to slot 0 for return values. The new wrenSetSlot___() functions allow writing to any slot index, making them general-purpose for manipulating the foreign call stack. This change updates all call sites, removes the dedicated returns test suite in favor of slots tests, and adjusts the VM internals to treat slot 0 as the return position rather than using a separate return mechanism.
