build: Add npm prepare script

[aloisklink]

Adds an npm prepare script to package.json

npm prepare runs automatically when doing npm install from a git dependency.

This will allow users to use dayjs with npm directly from the git repo, e.g. with:

npm install https://github.com/iamkun/dayjs.git

(feel free to test it by installing this branch, with npm install https://github.com/aloisklink/dayjs.git#add-npm-prepare-script)

It should be helpful for testing dayjs, as well as if people want to use bugfixes before dayjs pushes releases to NPM.

See https://docs.npmjs.com/cli/v7/using-npm/scripts for more details about npm prepare.

[iamkun]

I'm not sure if there will be some side effects or not. I'll check the docs later.