Allow override of "target" for ES2016+ features in Custom Server

https://github.com/blitz-js/blitz/blob/main/packages/blitz/src/cli/utils/next-utils.ts#L58
```
const getEsbuildOptions = (): esbuild.BuildOptions => {
return {
...
target: "es6",
...
}
}
```

The current implementation of getEsbuildOptions function hardcodes the target setting to "es6". This setting restricts the server from utilizing JavaScript features introduced in ES2016 and later versions. It would be beneficial to provide a mechanism that allows overriding the target option to support modern JavaScript features in a custom server environment.