Manager builder: support custom conditions

[jackw]

Is your feature request related to a problem? Please describe.

tldr; I'd like to be able to customise the conditions property in the builder-manager esbuild config.

I'm looking to use export custom conditions in a monorepo to resolve from source during dev/builds without the need to build all the packages first or impact how the public npm packages are consumed outside the repo. e.g.

// packages/pkgA/package.json
{
  main: './dist/cjs/index.cjs',
  module: './dist/esm/index.mjs',
  types: './dist/types/index.d.ts'
  exports: {
    '.': {
      '@app/src: './src/index.ts',
      'types': './dist/types/index.d.ts',
      'import': './dist/esm/index.mjs',
      'require': './dist/cjs/index.cjs'
    }
  }
 }

Like this I can update tooling to use the @app/src custom condition and we don't need to build all packages up front during dev / lint / etc.

However I cannot find a way to push the @app/src condition into the manager esbuild config which I need to do as the projects storybook manager.ts imports from pkgA.

Describe the solution you'd like

I'm not really sure how best to solve this as the manager builder package and it's API aren't intended for public consumption. There's a couple of env vars already in use in the esbuild config for browser targets and supported features. Maybe we could add an env var for this?

Describe alternatives you've considered

I'm currently using yarn patch to add the custom condition to storybook-core on install. It works but it's far from ideal considering updating dependencies.

diff --git a/dist/builder-manager/index.js b/dist/builder-manager/index.js
index 3d7f9b213dae1801bda62b31db31b9113e382ccd..212501c63d20146c29db63fb0f6300c6779eecb5 100644
--- a/dist/builder-manager/index.js
+++ b/dist/builder-manager/index.js
@@ -1970,7 +1970,7 @@ var pa = /^\/($|\?)/, G, C, xt = /* @__PURE__ */ o(async (e) => {
     bundle: !0,
     minify: !0,
     sourcemap: !1,
-    conditions: ["browser", "module", "default"],
+    conditions: ["@app/src", "browser", "module", "default"],
     jsxFactory: "React.createElement",
     jsxFragment: "React.Fragment",
     jsx: "transform",

Are you able to assist to bring the feature to reality?

yes, I can

Additional context

No response