[VS Code] Add Walkthrough to Setup Duo

Problem to solve

There are several steps that the user has to finish to set up code suggestions correctly - see !1629 (merged).

We require people to read through documentation and follow a series of steps.

Proposal

Utilise the VS Code Walkthrough

Walkthrough is a series of interactive "slides" where we can guide the user through any number of steps and we can even check "exit criteria" for steps like

• has the user added an account?
• has the user run a command?
• have they changed a setting?

Further details

Example from GitLens

walkthrough

Links / references

• Here is a sample extension: https://github.com/microsoft/vscode-extension-samples/tree/main/getting-started-sample
• docs https://code.visualstudio.com/api/references/contribution-points#contributes.walkthroughs