Feature request: Evaluate code during build using a prebuild evaluation function from esbuild

Hi, I have a feature request!

> Evaluate code during build using a prebuild evaluation function from esbuild

I would love a feature where I could import a special function from esbuild, say `prebuild`, and use it to perform custom build tasks, like [constant folding](https://en.wikipedia.org/wiki/Constant_folding), [loop optimization](https://en.wikipedia.org/wiki/Loop_optimization), string injection and [other optimizations](https://en.wikipedia.org/wiki/Optimizing_compiler).

The `prebuild` function would take in a function as the only argument, with some data about the current build. For example the version number from the projects package.json file.

The function provided to `prebuild` would run during build, with all the typescript intellisense one would expect, and maybe it must return a string, or at least something serializable.

```ts
import React from 'react'
import {prebuild} from 'esbuild'

export const MyComponent = () => {

return (