remove typechecks from cache on file close

[TheAngryByrd]

WHAT

🤖 Generated by Copilot at 9e9131a

This pull request adds a feature to the LSP server to forget documents that are no longer needed by the F# compiler service checker. It introduces a new function RemoveFileFromCache in CompilerServiceInterface.fs and calls it in AdaptiveServerState.fs when a document is closed or deleted.

🤖 Generated by Copilot at 9e9131a

Sing, O Muse, of the clever LSP server, the swift helper of code
That learned to forget the documents of old, when they vanished from the node
By the skill of the FSharpCompilerServiceChecker, the wise and powerful guide
That purged its cache of the useless files, like Zeus hurling thunder from his side

📝🧹🔥

WHY

Memory optimization

HOW

🤖 Generated by Copilot at 9e9131a

• Add a new function RemoveFileFromCache to the FSharpCompilerServiceChecker type that removes a file path from the checker's internal cache (link,link)
• Call the RemoveFileFromCache function in the forgetDocument function of the AdaptiveState type to update the LSP server state when a document is closed or deleted (link)
• Remove an unused open statement for the System.IO namespace from the FsAutoComplete namespace (link)