`parse` crashes when text is unicode

The following code crashes with `erlang:error(Badarg)`:
```gleam
let decoder =
decode.field("#text", decode.string, decode.success)
|> decode.field("unicode_test", _, decode.success)

assert Ok("β") == echo xml2map.parse("β", decoder)
```
I was able to patch it by using `unicode:characters_to_binary/1` instead of `list_to_binary/1` in `xml2map_ffi:value_to_bin/1`:
```diff
value_to_bin(Value) when is_list(Value) ->
- list_to_binary(Value);
+ unicode:characters_to_binary(Value);
```

Start at xml2map_ffi:value_to_bin/1, where the issue identifies the failing conversion, and reproduce the provided unicode XML assertion through xml2map.parse. Done means the assertion returns Ok("β") without an Erlang badarg error; check the surrounding FFI behavior for regressions.

Written by the indexing model from the issue text.