🎨 Add a prepare script

- makes the package installable from a git branch
(npm and equivalents run prepare on install in this case)

package.json

@@ -83,6 +83,7 @@
     "test:deno": "concurrently --success first -k 'npm run mock:wait && deno test --allow-net ./test/deno/wretch_test.ts' 'npm run mock'",
     "changelog": "conventional-changelog -p wretch -i CHANGELOG.md -s -r 0",
     "doc": "typedoc --out docs/api --readme none --excludeInternal src/index.ts 'src/addons/*.ts' 'src/middlewares/*.ts'",
+    "prepare": "npm run build",
     "prepare:release": "npm run start && npm run doc && npm run changelog"
   },
   "author": "Julien Elbaz",