fix: Add slash character for path.parse to support nested slash environment variables

[nooop3]

This library cannot parse successfully if the environment variables contain a slash and have nested values.
For example:

use config::Config;
use serde::Deserialize;

#[derive(Debug, Deserialize, PartialEq)]
struct ChildValue {
    v: u8,
}

#[derive(Debug, Deserialize, PartialEq)]
struct SlashNested {
    #[serde(rename = "slash/nested")]
    nested: ChildValue,
}

fn main() {
    let env = config::Environment::with_prefix("APP").separator("_");
    std::env::set_var("APP_slash/nested_v", "42");

    let config = Config::builder().add_source(env).build().unwrap();
    println!("{:?}", config.cache.to_string());

    let app: SlashNested = config.try_deserialize().unwrap();

    assert_eq!(
        app,
        SlashNested {
            nested: ChildValue { v: 42 }
        }
    );

    std::env::remove_var("APP_slash/nested_v");
}

Expected behavior:

• parse successfully with log "{ slash/nested => { v => 42, }, }"

Current behavior:

• parse failed(panicked) with log "{ slash/nested.v => 42, }"