Linter: doesn't work with stdin

[Modified into a bug report from the previous feature request; I don't think we actually need a `--config` argument to specify the location of a `.editorconfig`.]

`gdscript-formatter lint` doesn't work in `stdin` mode; I couldn't tell if this was intentional or not:

```sh
# Guessed from the docs/--help output:

$ cat dice.gd | gdscript-formatter lint
Error: No GDScript files found in the arguments provided. Please provide at least one .gd file or directory containing .gd files.

# Another CLI convention:

$ cat dice.gd | gdscript-formatter lint -
Error: No GDScript files found in the arguments provided. Please provide at least one .gd file or directory containing .gd files.
```

Maybe a `--stdin` argument to force this? I couldn't tell from the code, I can't Rust yet.

Start by tracing how the `gdscript-formatter lint` command parses file arguments and handles standard input, using the documented and `--help` invocation forms in the report. Confirm the intended stdin convention, then verify that piped GDScript is linted successfully without the current “No GDScript files found” error.

Written by the indexing model from the issue text.