Add NuGet package guidance to CS1061 compiler error documentation

[Copilot]

This PR adds general guidance to help developers find missing NuGet packages when encountering CS1061 compiler errors, addressing the issue where specific packages like Microsoft.AspNetCore.Identity.UI are needed but not documented.

Changes Made

Added a fourth correction step to the CS1061.md documentation that directs users to search NuGet.org when they suspect a missing member should exist but doesn't. This provides a general solution for finding appropriate NuGet packages without maintaining a comprehensive list of specific packages.

The new guidance follows the existing documentation pattern and aligns with the maintainer feedback to provide general directions rather than specific package references.

Before

The documentation only provided three ways to correct CS1061 errors:

1. Check spelling
2. Add missing member to class
3. Use extension method

After

Now includes a fourth option:
4. Search NuGet.org for packages containing the missing member

This change helps developers like the issue reporter who needed Microsoft.AspNetCore.Identity.UI but provides a scalable solution that works for any missing package scenario.

Also updated .gitignore to prevent accidentally committing node_modules/ from temporary tooling.

Fixes #37017.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

---

Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/compiler-messages/cs1061.md	docs/csharp/language-reference/compiler-messages/cs1061

[learn-build-service-prod]

PoliCheck Scan Report

The following reports lists PoliCheck issues in PR files. Before you merge the PR, you must fix all severity-1 issues. Other issues are also a high priority. The AI Review Details column lists suggestions for either removing or replacing the terms. If you find a false positive result, mention it in a comment that you post to the PR. The feedback helps reduce false positive instances in future scans.

✅ No issues found

More information about PoliCheck

Information: PoliCheck | Severity Guidance | Term
For any questions: Try searching the learn.microsoft.com contributor guides or post your question in the Learn support channel.

[BillWagner]

@copilot One change, and then this is ready.

[learn-build-service-prod]

PoliCheck Scan Report

The following report lists PoliCheck issues in PR files. Before you merge the PR, you must fix all severity-1 issues. Other issues are also a high priority. The AI Review Details column lists suggestions for either removing or replacing the terms. If you find a false positive result, mention it in a PR comment and include this text: #policheck-false-positive. This feedback helps reduce false positives in future scans.

✅ No issues found

More information about PoliCheck

Information: PoliCheck | Severity Guidance | Term
For any questions: Try searching the learn.microsoft.com contributor guides or post your question in the Learn support channel.

[BillWagner]

This now LGTM.

[learn-build-service-prod]

PoliCheck Scan Report

The following report lists PoliCheck issues in PR files. Before you merge the PR, you must fix all severity-1 issues. Other issues are also a high priority. The AI Review Details column lists suggestions for either removing or replacing the terms. If you find a false positive result, mention it in a PR comment and include this text: #policheck-false-positive. This feedback helps reduce false positives in future scans.

✅ No issues found

More information about PoliCheck

Information: PoliCheck | Severity Guidance | Term
For any questions: Try searching the learn.microsoft.com contributor guides or post your question in the Learn support channel.