feat: refactor into modules (#6)

Moves the various stages into their own modules and simplifies the main
entrypoint because of that.

Also adds a `bin` to the `package.json`.

package.json

@@ -8,6 +8,10 @@
     "lib",
     "!lib/test"
   ],
+  "bin": {
+    "module-replacements": "./lib/bin.js",
+    "mr": "./lib/bin.js"
+  },
   "scripts": {
     "clean:build": "premove lib",
     "clean:test": "premove coverage",

src/bin.ts

@@ -0,0 +1,4 @@
+#!/usr/bin/env node
+import {run} from './main.js';
+
+run();