[Repo Assist] fix: add DotRush (nromanov.dotrush) as a recognized C# extension alternative

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Summary

Closes #2119

DotRush is a lightweight C# extension for VS Code (and compatible editors like Cursor) that users have requested as an alternative to ms-dotnettools.csharp, anysphere.csharp, and muhammad-sammy.csharp.

Problem

Previously, any user who had only DotRush installed would get a blocking error at Ionide startup:

Ionide requires one of the following C# extensions to be installed: ms-dotnettools.csharp, anysphere.csharp or muhammad-sammy.csharp

…and Ionide would refuse to activate entirely. All F# language features (IntelliSense, type checking, project loading) were unavailable, even though DotRush can satisfy the underlying dependency.

Change

Add "nromanov.dotrush" to the requiredExtensions list in src/fsharp.fs:

// Before
let private requiredExtensions =
    [ "ms-dotnettools.csharp"   // VSCode C# extension
      "anysphere.csharp"        // Cursor C# extension
      "muhammad-sammy.csharp" ] // Free/Libre C# extension

// After
let private requiredExtensions =
    [ "ms-dotnettools.csharp"   // VSCode C# extension
      "anysphere.csharp"        // Cursor C# extension
      "muhammad-sammy.csharp"   // Free/Libre C# extension
      "nromanov.dotrush" ]      // DotRush C# extension

This list is only used to decide whether to show the "please install a C# extension" warning — it has no effect on other functionality.

Notes

• Language features: All Ionide F# language features will work normally for DotRush users after this change.
• Debugging: DotRush would need to register a coreclr-compatible debug adapter for F# debugging to work. If it does, debugging will work seamlessly; if not, users would need a separate C# extension for debugging. Either way, this is better than blocking all language features.
• extensionPack in release/package.json: To also suggest DotRush as a co-install option, release/package.json's extensionPack would need updating too — but that file requires maintainer review as it affects the published VS Code marketplace metadata.

Test Status

Build verified passing (dotnet fable src/Ionide.FSharp.fsproj) — only pre-existing deprecation warnings, no new errors.

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