Implement #:package manipulation for file-based apps

[jjonescz]

Part of #49200. This PR implements dotnet package add and dotnet package remove.

When more logic is shared with NuGet, more package-related commands can be implemented and some of the logic here simplified. See NuGet/Home#14390.

I've modeled the commands around their project-based counterparts. package remove is relatively simple - corresponding #:package directives are removed from the C# file, Directory.Packages.props is not considered at all. package add needs to update Directory.Packages.props if Central Package Management is enabled. It also needs to perform restore. If no version is specified by the user on the command-line, it needs to determine the latest version and put that into the package reference directive.

[Copilot]

Pull Request Overview

This PR extends the dotnet package commands to support file-based C# apps by introducing a --file option, updating parsing/completion, and adding source‐editor logic to manipulate #:package directives.

• Introduces --file alongside --project in parsers and completion scripts
• Implements add/remove of #:package directives in FileBasedAppSourceEditor and wiring in PackageAddCommand / PackageRemoveCommand
• Adds extensive unit tests and updates documentation for the new file-based scenarios

Reviewed Changes

Copilot reviewed 57 out of 57 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/Cli/dotnet/Commands/Package/PackageCommandParser.cs	Adds FileOption and path‐processing logic for file-based apps
src/Cli/dotnet/Commands/Package/Add/PackageAddCommand.cs	Implements file‐based add logic, including CPM and restore support
src/Cli/dotnet/Commands/Package/Remove/PackageRemoveCommand.cs	Implements file‐based remove logic and reports removed directives
src/Cli/dotnet/Commands/Run/VirtualProjectBuildingCommand.cs	Refactors diagnostics handling and directive loading for files
src/Cli/dotnet/Commands/Run/FileBasedAppSourceEditor.cs	New editor for inserting/removing #:package directives
src/Cli/dotnet/Commands/Run/RunCommand.cs	Updates error message constant for invalid option combinations

Comments suppressed due to low confidence (1)

src/Cli/dotnet/Commands/Run/FileBasedAppSourceEditor.cs:105

• Typo in variable name addAfer—it should be spelled addAfter to match the intended meaning.

        CSharpDirective? addAfer = null;