Feature request: running default loaders from onLoad hook

Hi, First of all I want to thank you for such a fast and customizable library. It allows me to do a lot of things.

## Problem
At the moment I am working on generating static websites, the source code of which is written in typescript and react.
As an input point I need to set some `.tsx` file, which should turn into an `.html` file on output. I can do this by redirecting the path in the `onResolve` hook. However, I need to import the styles file and the original `.ts` file. It seems that I can't just read the `.ts` file with `fs`, because it has to be processed with the standard ts loader. But if I return that ts file in `onLoad`, that's what goes into the final output.

At the moment I have to subscribe to the `onEnd` hook and do my manipulations in it, which gives me 2 files: `.ts` and `.html`, but I would like to have only one.

## Suggestion
What I think can be done: by analogy with `build.resolve` make `build.load`, which will call the hook `onResolve`, then `onLoad` and then return the final result. So I'll be able to do something like this:
```javascript
build.onResolve({ filter: /\.tsx$/ }, (args) => {
if (args.kind === "entry-point") {
return {
path: htmlPath(args),
namespace: "@namespace-html",
pluginData: {
resolveDir: args.resolveDir,
originalFile: args.path,
},
};
}
});

build.onLoad(
{ filter: /.+/, namespace: "@namespace-html" },
async (args) => {
const originalEntrypointAsString = await build.load(
path.join(args.pluginData.resolveDir, args.pluginData.originalFile)
);

const moduleResult = await import(
toDataUrl(originalReactComponentString)
);

const reactElement = moduleResult.reactElement;
const cssPath = moduleResult.cssPath; // already moved to assets

return {
contents: ReactDOM.renderToStaticMarkup(reactElement).replace("{CSS}", cssPath),
loader: "file"
};
}
);
```

I'm also considering calling `build.esbuild.transorm()`, but in that case I get problems with assets.
Calling `build.esbuild.build()` would also cause issues with assemblies and break the links between builds, since it would actually be calling N+1 isolated builds.

Start with esbuild's plugin onResolve and onLoad hooks and compare the requested behavior with the existing build.resolve analogy. Determine how a build.load-style API could invoke the default loaders while preserving assets and links between builds; done means the original entry point produces only the generated HTML without a separate TypeScript output.

Written by the indexing model from the issue text.