Improve where file-level directives are added

[jjonescz]

Description

dotnet add app.cs package will now insert #:package directives:

• After #! directives (first commit) because #! needs to always be first in the file.
• Sorted (second commit) to match the behavior of dotnet add project.csproj package.

Customer impact

This was discovered by me while dogfooding the feature.

Regression

No.

Risk

Low. This is just a tooling update, scoped to the new feature (file-based apps). Unit tests verify the behavior.

[Copilot]

Pull Request Overview

This PR improves the placement and ordering of file-level directives in C# script files by handling shebang directives and implementing sorted insertion of named directives.

• Directives are now placed after shebang (#!) directives to respect script file structure
• Named directives (like packages) are inserted in alphabetical order for better organization

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/Cli/dotnet/Commands/Run/FileBasedAppSourceEditor.cs	Implements sorting logic for named directives and shebang handling in directive placement
test/dotnet.Tests/CommandTests/Run/FileBasedAppSourceEditorTests.cs	Adds test cases for sorted directive insertion and shebang directive handling

[jjonescz]

@RikkiGibson for a review, thanks