build: dual CJS/ESM output, ES2020 target, Node 18 floor

"exports" gains "import" and "types" next to "require": import "alex2node" failed with
ERR_PACKAGE_PATH_NOT_EXPORTED. scripts/build.mjs compiles src/ to dist/cjs, dist/esm and dist/types in a staging
directory and swaps them in only when both compiler runs passed. "main" and "types" follow for resolvers that do
not read "exports". Target ES6 -> ES2020, "engines": node >= 18, relative imports carry ".js" for Node's ESM loader.
uuid gives way to crypto.randomUUID(); mqtt is the only runtime dependency left.

Tests load the package by name. New: both entry points, the modules the builds import, dist/ equal to a fresh
build, a round trip on the ES module build, and two compiled type fixtures in place of the .d.ts regexes.
22 tests pass in 5.7 s. dist/: 16 files, 79,711 B -> 25 files, 132,291 B; the npm tarball is 25,125 B.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tsconfig.json

@@ -1,14 +1,12 @@
 {
-    "compilerOptions": {
-      "target": "ES6",
-      "module": "CommonJS",
-      "moduleResolution": "node",
-      "outDir": "./dist",
-      "declaration": true,
-      "esModuleInterop": true,
-      "strict": true,
-      "skipLibCheck": true
-    },
-    "include": ["src/**/*.ts"]
-  }
-  
+  "compilerOptions": {
+    "target": "ES2020",
+    "module": "CommonJS",
+    "moduleResolution": "node",
+    "esModuleInterop": true,
+    "strict": true,
+    "skipLibCheck": true,
+    "noEmit": true
+  },
+  "include": ["src/**/*.ts"]
+}