FeatureRequest: `_FILE` suffix to env vars

I believe adding support to `_FILE` suffixes would be a great addition to this library. There are cases where we may want to load data not from the env, but from a secret file, like `/run/secrets/`, and although the library already supports the `,file` option, that would require the user to have a struct like:

```go
type Config struct {
Password `env:"PASSWORD"`
PasswordFile `env:"PASSWORD_FILE,file"`
}
```

My proposal is to have an option that enables this feature globally, or by field:

**By Field:**

```go
type Config struct {
Password `env:"PASSWORD,allow_file_suffix"`
}
```

**Global:**

```go
type Config struct {
Password `env:"PASSWORD"`
}
conf := Config{}
env.ParseWithOptions(&conf, env.Options{
AllowFileSuffix: True
})
```

If you find it interesting to add, I would love to work on the feature myself.

Start by reading the existing `,file` option and the `env.ParseWithOptions` and `env.Options` entry points. Compare the proposed field tag `allow_file_suffix` with the global `AllowFileSuffix` option, then trace how environment names and secret files are resolved. Done means both configuration forms support the `_FILE` convention without breaking existing `,file` behavior, with tests covering each form.

Written by the indexing model from the issue text.