add nushell script to create new Rust projects for each challenge

This almost certainly took more time to write than it will ever save me,
but at least I got to learn a new scripting language.

scripts/next.nu

···
+#!/usr/bin/env nu
+
+let current = ls | where name =~ \d\d and type == dir | get name | sort | last | default "00"
+let part2 = $current | path join part2
+
+def cargo-config [current part] {
+	{ package: { name: $"aoc25-($current)-($part)" version: "1.0.0" edition: "2024" } }
+	| to toml
+}
+
+if (($current == "00") or ($part2 | path exists)) {
+	let next = $current | into int | $in + 1 | into string | fill -a r -c '0' -w 2
+	let part1 = $next | path join part1
+
+	mkdir ($part1 | path join src)
+	cargo-config $next 1 | save ($part1 | path join Cargo.toml)
+
+	r#'use std::fs::read_to_string;
+
+fn main() {
+    read_to_string("../input.txt").expect("failed to open input file");
+}'# | save ($part1 | path join src main.rs)
+} else {
+	mkdir $part2
+	cargo-config $current 2 | save ($part2 | path join Cargo.toml)
+
+	cp -r ($current | path join part1 src) ($part2 | path join src)
+}