Add --file and --startup-file options

[jjonescz]

Resolves #38159.

• I've read the guidelines for contributing and seen the walkthrough
• I've posted a comment on an issue with a detailed description of how I am planning to contribute and got approval from a member of the team
• The code builds and tests pass locally (also verified by our automated build checks)
• Commit messages follow this format:

        Summary of the changes
        - Detail 1
        - Detail 2

        Fixes #bugnumber

• Tests for the changes have been added (for bug fixes / features)
• Code follows the same patterns and style as existing code in this repo

[Copilot]

Pull request overview

Adds --file and --startup-file options to dotnet-ef, aligning option naming with file-based apps usage while keeping existing --project / --startup-project behavior. This primarily affects how the tool resolves project/startup inputs and how those options are described in help text/resources.

Changes:

• Introduced --file / --startup-file options and routed them through project/startup path resolution.
• Added mutual-exclusion validation between the primary options and their new aliases.
• Updated/added localized resource strings and generated designer accessors for the new option descriptions and error message.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/dotnet-ef/RootCommand.cs	Adds new option wiring and ResolveOption helper enforcing mutual exclusivity and resolution precedence.
src/dotnet-ef/ProjectOptions.cs	Defines the new --file and --startup-file command-line options.
src/dotnet-ef/Properties/Resources.resx	Adds descriptions for new options and a new mutually-exclusive-options error message; updates existing option descriptions.
src/dotnet-ef/Properties/Resources.Designer.cs	Adds strongly-typed accessors for the new resource strings.

Files not reviewed (1)

• src/dotnet-ef/Properties/Resources.Designer.cs: Language not supported