Add parse_env attribute to confique::Config macro

Since there is no established format for representing collections within env variables, to enable their support, the feature with a custom parser attribute added.

Add helpers for parsing env variables into collections

Improve rustdoc for `env_utils` functions

Improve `parse_env` example

examples/parse_env.rs

@@ -0,0 +1,83 @@
+#![allow(dead_code)]
+
+use confique::{
+    env_utils::{
+        to_collection_by_char_separator, to_collection_by_comma, to_collection_by_semicolon,
+    },
+    Config,
+};
+use std::{collections::HashSet, num::NonZeroU64, path::PathBuf, str::FromStr};
+
+#[derive(Debug, Config)]
+struct Conf {
+    #[config(
+        env = "PATHS",
+        parse_env = to_collection_by_comma
+    )]
+    paths: HashSet<PathBuf>,
+    #[config(
+         env = "PORTS",
+         parse_env = to_collection_by_semicolon
+    )]
+    ports: Vec<u16>,
+    #[config(
+         env = "NAMES",
+         parse_env = to_collection_by_char_separator::<'|', _, _>
+    )]
+    names: Vec<String>,
+    #[config(
+         env = "TIMEOUT",
+         parse_env = NonZeroU64::from_str,
+    )]
+    timeout_seconds: NonZeroU64,
+    #[config(
+         env = "FORMATS",
+         parse_env = parse_formats,
+    )]
+    formats: Vec<Format>,
+}
+
+#[derive(Debug, serde::Deserialize)]
+enum Format {
+    Env,
+    Toml,
+    Json5,
+    Yaml,
+}
+
+#[derive(Debug)]
+enum Error {}
+
+fn parse_formats(input: &str) -> Result<Vec<Format>, Error> {
+    let mut result = Vec::new();
+
+    if input.contains("toml") {
+        result.push(Format::Toml);
+    }
+    if input.contains("env") {
+        result.push(Format::Env);
+    }
+    if input.contains("yaml") {
+        result.push(Format::Yaml);
+    }
+    if input.contains("json5") {
+        result.push(Format::Json5);
+    }
+
+    Ok(result)
+}
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    std::env::set_var("PATHS", "/bin/ls,/usr/local/bin,/usr/bin/ls");
+    std::env::set_var("PORTS", "8080;8888;8000");
+    std::env::set_var(
+        "NAMES",
+        "Mervinc Harmon|Alfreda Valenzuela|Arlen Cabrera|Damon Rice|Willie Schwartz",
+    );
+    std::env::set_var("TIMEOUT", "100");
+    std::env::set_var("FORMATS", "json5,yaml;.env");
+
+    println!("{:#?}", Conf::builder().env().load()?);
+
+    Ok(())
+}