feat(js): ignore scoped export conditions (#32063)

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There is an emerging pattern of using scoped custom export conditions
for pointing to source. See:
- [Announcing TypeScript
5.7](https://devblogs.microsoft.com/typescript/announcing-typescript-5-7/#path-rewriting-for-relative-paths:~:text=As%20a%20result%2C%20if%20you%E2%80%99ve%20been%20using%20a%20workspace%2Dstyle%20layout%20with%20multiple%20packages%20referencing%20each%20other%2C%20you%20might%20need%20to%20use%20conditional%20exports%20with%20scoped%20custom%20conditions%20to%20make%20this%20work%3A)
- [Live types in a TypeScript
monorepo](https://colinhacks.com/essays/live-types-typescript-monorepo#:~:text=Here%27s%20how%20a%20custom%20condition%20might%20look%20in%20your%20package.json.%20There%27s%20nothing%20special%20about%20the%20string%20%22%40colinhacks/zod%22%20here!%20It%20could%20be%20anything.)
- [Loading from
Source](https://github.com/isaacs/tshy?tab=readme-ov-file#loading-from-source)

## Current Behavior
<!-- This is the behavior we have today -->

When including an export condition that points to the source code the
typescript plugin does not add a build target. However, if this export
condition is named `development` the build target is added.

## Expected Behavior
<!-- This is the behavior we should expect with the changes in this PR
-->

Scoped export conditions pointing at source should not remove the build
target.

## Related Issue(s)
<!-- Please link the issue being fixed so it gets closed when this is
merged. -->

Author: aidant

packages/js/src/plugins/typescript/util.ts

@@ -113,7 +113,8 @@ export function isValidPackageJsonBuildConfig(
           return !isPathSourceFile(dotExport);
         } else if (typeof dotExport === 'object') {
           const hasMatch = Object.entries(dotExport).some(([key, value]) => {
-            if (key === 'types' || key === 'development') return false;
+            if (key === 'types' || key === 'development' || key.startsWith('@'))
+              return false;
             return typeof value === 'string' && isPathSourceFile(value);
           });
           return !hasMatch;
@@ -193,7 +194,11 @@ export function isValidPackageJsonBuildConfig(
     } else if (typeof value === 'object') {
       return Object.entries(value).some(([currentKey, subValue]) => {
         // Skip types and development conditions
-        if (currentKey === 'types' || currentKey === 'development') {
+        if (
+          currentKey === 'types' ||
+          currentKey === 'development' ||
+          currentKey.startsWith('@')
+        ) {
           return false;
         }
         if (typeof subValue === 'string') {