Add API to query the original string

I was looking at `PatternElement` and while going through, I realized that there was no method to query the original string or to convert to such a string. Would it be possible to add such an API, which would allow things like:

```rust
let x: PatternElement;

x.to_fluent_message();
```

No need to go through the multiple enums level if we're only interested into rebuilding a FTL file.