feat: add output flag (#13)

## Description

The lambda CLI previously only wrote output to stdout using shell redirection.
This PR adds support for writing results to files using the `-o` flag.
This is implemented using a new `Destination` interface that mirrors the existing `Source` pattern.

Changes:
- Added `Destination` interface with `StdoutDestination` and `FileDestination` implementations.
- Added `-o` flag to CLI argument parser for output file specification.
- Updated `Config` to use `Destination` instead of direct output handling.
- Refactored main to use `Destination.Write()` for result output.
- Updated Makefile targets (`run`, `profile`, `explain`) to use `-o` flag instead of shell redirection.

### Decisions

The `-o` flag defaults to stdout when not specified or when set to `-`.
This maintains backward compatibility while providing explicit file output capability.

## Benefits

- Cleaner command-line interface without shell redirection.
- Symmetric design with `Source` interface for input.
- More portable across different shells and environments.
- Explicit output handling improves code clarity.

## Checklist

- [x] Code follows conventional commit format.
- [x] Branch follows naming convention (`<type>/<description>`).
- [ ] Tests pass (if applicable).
- [ ] Documentation updated (if applicable).

Reviewed-on: #13
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

cmd/lambda/lambda.go

@@ -1,7 +1,6 @@
 package main
 
 import (
-	"fmt"
 	"os"
 
 	"git.maximhutz.com/max/lambda/internal/cli"
@@ -72,5 +71,7 @@ func main() {
 	process.Run()
 
 	// Return the final reduced result.
-	fmt.Println(lambda.Stringify(compiled))
+	result := lambda.Stringify(compiled)
+	err = options.Destination.Write(result)
+	cli.HandleError(err)
 }