feat: Add a prepare script.

[trusktr]

Summary

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.

For example, in package.json the following,

        "docsify": "docsifyjs/docsify#prepare-script",

tells npm to get docsify from the branch prepare-script from http://github.com/docsifyjs/docsify.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

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• DO NOT include files inside lib directory.

[jamesgeorge007]

Please resolve the conflicts.