feat: add Fiber.try() method for catching errors across fiber boundaries

Implement a new `Fiber.try()` method that allows a calling fiber to catch runtime errors from a called fiber. When a fiber is invoked with `try()` instead of `call()`, any runtime error that occurs in the called fiber is returned as a string to the caller rather than terminating the program with a stack trace. The `callerIsTrying` flag on `ObjFiber` tracks this state, and the `runtimeError` function now checks this flag to redirect the error to the caller's stack. Also adds `Fiber.error` getter to retrieve the error string from a failed fiber, changes `Fiber.isDone` to return `true` for errored fibers, and migrates the fiber error field from a raw `char*` to an `ObjString*` for proper memory management. Includes comprehensive test cases for try behavior, re-entry prevention, and error state queries.

test/fiber/error.wren

@@ -0,0 +1,7 @@
+var fiber = new Fiber {
+  "s".unknown
+}
+
+IO.print(fiber.error) // expect: null
+IO.print(fiber.try)   // expect: String does not implement method 'unknown'.
+IO.print(fiber.error) // expect: String does not implement method 'unknown'.

test/fiber/is_done_after_error.wren

@@ -0,0 +1,6 @@
+var fiber = new Fiber {
+  "s".unknown
+}
+
+fiber.try
+IO.print(fiber.isDone) // expect: true

test/fiber/try.wren

@@ -0,0 +1,10 @@
+var fiber = new Fiber {
+  IO.print("before")
+  true.unknownMethod
+  IO.print("after")
+}
+
+IO.print(fiber.try)
+// expect: before
+// expect: Bool does not implement method 'unknownMethod'.
+IO.print("after try") // expect: after try

test/fiber/try_direct_reenter.wren

@@ -0,0 +1,7 @@
+var fiber
+
+fiber = new Fiber {
+  fiber.try // expect runtime error: Fiber has already been called.
+}
+
+fiber.call

test/fiber/try_indirect_reenter.wren

@@ -0,0 +1,12 @@
+var a
+var b
+
+a = new Fiber {
+  b.try // expect runtime error: Fiber has already been called.
+}
+
+b = new Fiber {
+  a.call
+}
+
+b.call

test/fiber/try_when_done.wren

@@ -0,0 +1,6 @@
+var fiber = new Fiber {
+  IO.print("try")
+}
+
+fiber.try // expect: try
+fiber.try // expect runtime error: Cannot try a finished fiber.

test/fiber/try_without_error.wren

@@ -0,0 +1,8 @@
+var fiber = new Fiber {
+  IO.print("fiber")
+}
+
+IO.print("before")  // expect: before
+IO.print(fiber.try) // expect: fiber
+                    // expect: null
+IO.print("after")   // expect: after