feat!: redesigns vue upgrade, supports code migration

[haoqunjiang]

The basic idea

Inspired by https://nodepackjs.com/guide/app-migrations.html and https://github.com/angular/angular-cli/blob/master/docs/specifications/update.md

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The new vue upgrade command

Usage: upgrade [options] [package-name]

upgrade vue cli service / plugins

Options:
  -t, --to <version>    upgrade to a version that is not latest
  -r, --registry <url>  Use specified npm registry when installing dependencies

The old command is only an adapter for npm install and only takes care of core plugins.

After this change, third-party plugins can also benefit from this command.

In the future, we may also extend this utility for core package migrations (if possible) (e.g. vue upgrade vuex --to 4.0.0)

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The new MigratorAPI:

Basically the same as GeneratorAPI, except for one more method: fromVersion and it's triggered on plugin upgrade only.
Migrator implementation should be located under ./migrator directory.

The idea is to provide a minimal API interface for migration utilities and to be fully programmable/customizable.

Later I'd also like to add a few handy helper methods to both GeneratorAPI and MigratorAPI, such as api.move and api.remove.

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TODOs in this PR:

• clean the code (there're all kinds of TODOs and FIXMEs in it and many functionalities are broken)
• tests
• an example migrator (in the babel plugin)
• interactive "upgrade all" command
• documentation

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A few ideas with regard to the GeneratorAPI:

• api.move
• api.remove
• api.transformScript, utilizing jscodeshift and vue-jscodeshift-adapter
• api.patchJSON, utilizing golden-fleece don't feel it's necessary