Restructure folders and add day 3's solution

javascript/day_03/solution.mjs

@@ -0,0 +1,35 @@
+import { readFile } from "fs/promises";
+
+if (process.argv.length <= 2) {
+	console.error(`Needs more args >:(`).
+	process.exit(0);
+}
+
+const fileToRead = process.argv[2];
+const file = await readFile(fileToRead, `utf-8`);
+
+const pattern = /(?<cmd>mul|do|don't)\(((?<a>\d{1,3}),(?<b>\d{1,3}))?\)/g;
+
+const commands = file.matchAll(pattern);
+
+let part1Sum = 0;
+let part2Sum = 0;
+let enabled = true
+for (const command of commands) {
+	const { cmd, a, b } = command.groups;
+
+	switch (cmd) {
+		case "do": enabled = true; break
+		case "don't": enabled = false; break
+		case "mul": {
+			part1Sum += a * b;
+			if (enabled) {
+				part2Sum += a * b;
+			}
+			break;
+		}
+	}
+};
+
+console.log(`Part 1 Answer: ${part1Sum}`);
+console.log(`Part 2 Answer: ${part2Sum}`);