Make the CLI parser not barf on unknown tokens

[baronfel]

Per #952 unknown tokens cause exceptions. We should be more flexible and ignore them instead of hard erroring to make backwards-compatibility easier.

[TheAngryByrd]

I think https://learn.microsoft.com/en-us/dotnet/api/system.commandline.command.treatunmatchedtokensaserrors?view=system-commandline might be what we need to set. Otherwise we probably want to keep old flags around and create issues for clients when we deprecate them.

[github-actions]

🤖 This is an automated response from Repo Assist.

Good news — this issue appears to have already been resolved! In src/FsAutoComplete/Parser.fs, line 116 has:

// for back-compat - we removed some options and this broke some clients.
rootCommand.TreatUnmatchedTokensAsErrors <- false

This was exactly the fix suggested by @TheAngryByrd above (using TreatUnmatchedTokensAsErrors). Unknown CLI tokens are silently ignored rather than causing exceptions.

This issue can likely be closed. 🎉

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