feat: Add a prepare script.

This makes it possible to depend on the docsify package using a git commit hash instead of a version number in package.json, and causes the `lib` folder to be built in those cases, otherwise consumers will try to import from a folder that doesn't exist.

package.json

@@ -37,6 +37,7 @@
     "build:ssr": "node build/ssr.js",
     "build:cover": "node build/cover.js",
     "build": "rimraf lib themes/* && run-s build:js build:css build:css:min build:ssr build:cover",
+    "prepare": "npm run build",
     "pub:next": "cross-env RELEASE_TAG=next sh build/release.sh",
     "pub": "sh build/release.sh",
     "postinstall": "opencollective postinstall"