show copilot instructions prompt at startup

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Directly invoked by the user via "create Q# project" or "update Copilot instructions for Q#":

One-time prompt at startup, or indirectly invoked whenever our copilot tools get called:

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I'm not merging this yet because I want to refine this a little to work better with VS Code Next's support for instructions file. For one, we should be able to drop our own .github/instructions/qsharp.instructions.md in the workspace and avoid having to append to the global copilot-instructions.md . There are other things you can do with settings/user profiles but I can't make up my mind on where exactly we can hook in.

https://github.com/microsoft/vscode-docs/blob/vnext/docs/copilot/copilot-customization.md#specify-custom-instructions-in-settings

redid the whole PR

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This PR is ready with a new implementation, please review