feat: support multi-entry library build

[semiaddict]

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

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Underlying tools
• Other, please describe:

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

• Yes
• No

Other information:

This PR adds support to multi-entry library builds.
It does so by looking at Webpack's Entry Points config and processing the last file in each entry point in resolveLibConfig.

Example usage with vue.config.js:

```js
module.exports = {
    chainWebpack: (config) => {
        config.output.library({
          name: "MyLibrary",
          type: "assign-properties",
        });

        config.entryPoints.clear();
        config.entry("MyLibrary.EntryOne").add("./src/my-polyfill.js").add("./src/entry-one/main.js").end();
        config.entry("MyLibrary.EntryTwo").add("./src/entry-two/main.js").end();
    }
}

Related issue: #3922

[peynman]

Please merge this

[jincan39]

no more progress?

[yoyo837]

Any update?

[braddialpad]

Also interested in this. Had to make quite a workaround to get this to work.