feat: implement eager * operator for String and List to repeat content by integer count

Add `*(count)` method to both String and List classes in wren_core.wren, enabling eager repetition of string characters or list elements. The operator validates that count is a non-negative integer, aborting with a clear error message otherwise. For strings, concatenates the original string count times; for lists, builds a new list by adding all elements count times. Includes comprehensive test cases for normal, zero, and edge-case inputs, plus error tests for negative, non-integer, and non-numeric counts. Also refactors existing list plus tests to use inline expressions and adds a plus_not_iterable error test.

src/vm/wren_core.wren

@@ -131,6 +131,18 @@ class WhereSequence is Sequence {
 class String is Sequence {
   bytes { StringByteSequence.new(this) }
   codePoints { StringCodePointSequence.new(this) }
+
+  *(count) {
+    if (!(count is Num) || !count.isInteger || count < 0) {
+      Fiber.abort("Count must be a non-negative integer.")
+    }
+
+    var result = ""
+    for (i in 0...count) {
+      result = result + this
+    }
+    return result
+  }
 }
 
 class StringByteSequence is Sequence {
@@ -174,6 +186,18 @@ class List is Sequence {
     }
     return result
   }
+
+  *(count) {
+    if (!(count is Num) || !count.isInteger || count < 0) {
+      Fiber.abort("Count must be a non-negative integer.")
+    }
+
+    var result = []
+    for (i in 0...count) {
+      result.addAll(this)
+    }
+    return result
+  }
 }
 
 class Map {

src/vm/wren_core.wren.inc

@@ -133,6 +133,18 @@ static const char* coreModuleSource =
 "class String is Sequence {\n"
 "  bytes { StringByteSequence.new(this) }\n"
 "  codePoints { StringCodePointSequence.new(this) }\n"
+"\n"
+"  *(count) {\n"
+"    if (!(count is Num) || !count.isInteger || count < 0) {\n"
+"      Fiber.abort(\"Count must be a non-negative integer.\")\n"
+"    }\n"
+"\n"
+"    var result = \"\"\n"
+"    for (i in 0...count) {\n"
+"      result = result + this\n"
+"    }\n"
+"    return result\n"
+"  }\n"
 "}\n"
 "\n"
 "class StringByteSequence is Sequence {\n"
@@ -176,6 +188,18 @@ static const char* coreModuleSource =
 "    }\n"
 "    return result\n"
 "  }\n"
+"\n"
+"  *(count) {\n"
+"    if (!(count is Num) || !count.isInteger || count < 0) {\n"
+"      Fiber.abort(\"Count must be a non-negative integer.\")\n"
+"    }\n"
+"\n"
+"    var result = []\n"
+"    for (i in 0...count) {\n"
+"      result.addAll(this)\n"
+"    }\n"
+"    return result\n"
+"  }\n"
 "}\n"
 "\n"
 "class Map {\n"