Npm scripts: use "prepare" instead of "prepublishOnly"

When a prepare script is defined, npm and yarn will build uppy
automatically if it's being depended on through a git URL.
This is useful when downstream projects don't want to wait for
the next release to be tagged.

See npm/npm#3055 (comment)
See also yarnpkg/yarn#3553

package.json

@@ -168,7 +168,7 @@
     "web:update:frontpage:code:sample": "cd website && ./node_modules/.bin/hexo generate && cp -f public/frontpage-code-sample.html ./themes/uppy/layout/partials/frontpage-code-sample.html",
     "web": "npm-run-all web:clean web:build",
     "uploadcdn": "bin/upload-to-cdn.sh",
-    "prepublishOnly": "npm-run-all clean build",
+    "prepare": "npm-run-all clean build",
     "contributors": "githubcontrib --owner transloadit --repo uppy --cols 6 $([ \"${GITHUB_TOKEN:-}\" == \"\" ] && echo \"\" || echo \"--authToken ${GITHUB_TOKEN}\") --showlogin true --sortOrder desc",
     "contributors:save": "replace-x -m '<!--contributors-->[\\s\\S]+<!--/contributors-->' \"<!--contributors-->\n## Contributors\n\n$(npm run --silent contributors)\n<!--/contributors-->\" README.md"
   }