feat: implement resumable effect handlers

Add support for the `resume(value)` expression in effect handler
bodies. When resume is called, the value becomes the return value
of the effect operation, allowing handlers to provide values back
to the calling code.

Implementation:
- Add Resume(Value) variant to EvalResult
- Add in_handler_depth tracking to Interpreter
- Update Expr::Resume evaluation to return Resume when in handler
- Handle Resume results in handle_effect to use as return value
- Add 2 tests for resumable handlers

Example usage:
```lux
handler prettyLogger: Logger {
    fn log(level, msg) = {
        Console.print("[" + level + "] " + msg)
        resume(())  // Return Unit to the call site
    }
}
```

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

examples/handlers.lux

@@ -0,0 +1,45 @@
+// Demonstrating resumable effect handlers in Lux
+//
+// Handlers can use `resume(value)` to return a value to the effect call site
+// and continue the computation. This enables powerful control flow patterns.
+//
+// Expected output:
+// [INFO] Starting computation
+// [DEBUG] Intermediate result: 10
+// [INFO] Computation complete
+// Final result: 20
+
+// Define a custom logging effect
+effect Logger {
+    fn log(level: String, msg: String): Unit
+    fn getLogLevel(): String
+}
+
+// A function that uses the Logger effect
+fn compute(): Int with {Logger} = {
+    Logger.log("INFO", "Starting computation")
+    let x = 10
+    Logger.log("DEBUG", "Intermediate result: " + toString(x))
+    let result = x * 2
+    Logger.log("INFO", "Computation complete")
+    result
+}
+
+// A handler that prints logs with brackets and resumes with Unit
+handler prettyLogger: Logger {
+    fn log(level, msg) = {
+        Console.print("[" + level + "] " + msg)
+        resume(())
+    }
+    fn getLogLevel() = resume("DEBUG")
+}
+
+// Main function
+fn main(): Unit with {Console} = {
+    let result = run compute() with {
+        Logger = prettyLogger
+    }
+    Console.print("Final result: " + toString(result))
+}
+
+let output = run main() with {}