feat: add list clear and insert primitives with capacity growth and index validation

Extract inline list growth logic into `ensureListCapacity` helper and add `validateIndex` function supporting negative indices. Implement `list_clear` primitive that resets count to zero without freeing memory, and `list_insert` primitive that shifts elements right and inserts at validated position. Add comprehensive test suites for both operations covering empty lists, normal/negative indices, and return values.

test/list/clear.wren

@@ -0,0 +1,7 @@
+var a = [1, 2, 3]
+a.clear
+io.write(a)       // expect: []
+io.write(a.count) // expect: 0
+
+// Returns null.
+io.write([1, 2].clear) // expect: null