Updating Lifecycle Event Names to `onEvent`

The object returned by plugins represent the config, name, and the handlers that respond to lifecycle events.

These lifecycle events are events that happen during the course of a build. For example preBuild postBuild, preDeploy etc

Because these handler functions are responding to events, I think it would improve clarity of the plugin API to make the keys of event handler functions look more like an event handler.

For example, instead of prebuild using onPrebuild.

Changes

module.exports = function myPlugin(config) {
  return {
-   preBuild: () => {
+   onPreBuild: () => {
      // run thing on pre-build
    },
-   build: () => {
+   onBuild: () => {
      // run thing on build
    },
-   postBuild: () => {
+   onPostBuild: () => {
      // run thing on post build
    }
  }
}

I believe this would make it a little more clear that the keys beginning with on are bits of functionality that response to a particular lifecycle event.

I also think this will make it easier to evolve the plugin API without running into namespace clashes. E.g. (random example) if we wanted to use the build key in the future for some kind of config

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We can achieve this with a breaking change or non-breaking change.

The non-breaking change would require some mapping of existing keys & deprecation notices for plugins using the “older” non onXyz syntax

[DavidWells]

Lets go ahead and change this.

Part 2 before merging the code changes will be updating the plugin content we can edit. https://www.notion.so/netlify/Build-Plugins-Reference-Guide-2b5ed75d19254b5389f2713bfc0fef6a

---

Just to be clear, the expected behavior is migrating the lifecycle events in a backward compatible way.

E.g.

1. build would change to onBuild (etc)
2. Older plugins with build will still operate as normal but will log out the build lifecycle has been updated to onBuild, please update your plugins accordingly
3. At some future date we can retired the non onXyz lifecycle events

[ehmicky]

I think this is a good idea. 👍

Tagging @verythorough as this would require some documentation/articles updates.