Add options.onEvent, aiming to add event listeners to the build process.

[YangYongAn]

In the case in bundle mode , if multiple entries reference the same style file, the generated CSS file will repeatedly include this style file.

Here is a detailed example: In test/repeat-css, the components demo are referenced in both pages/main and pages/sub, and it's clear that the style file of demo appears multiple times in the dist output.

Therefore, I need to add event listeners (options.onEvent) during the build process to broadcast the input and output of each action in the build process through events. This will allow other plugins to further process the source data in the onEnd phase, such as separating CSS files into external files based on the import path.

[YangYongAn]

Although there are many diffs generated by the changes, and it seems like they are not related to the main functionality, this feature is indeed very useful to me.