esbuild 0.17.2

Add onDispose to the plugin API (#2140, #2205)

If your plugin wants to perform some cleanup after it's no longer going to be used, you can now use the onDispose API to register a callback for cleanup-related tasks. For example, if a plugin starts a long-running child process then it may want to terminate that process when the plugin is discarded. Previously there was no way to do this. Here's an example:
```
let examplePlugin = {
  name: 'example',
  setup(build) {
    build.onDispose(() => {
      console.log('This plugin is no longer used')
    })
  },
}
```
These onDispose callbacks will be called after every build() call regardless of whether the build failed or not as well as after the first dispose() call on a given build context.