Add configuration menu workflow

[atrakh]

This PR adds a new method for configuration the extension through an interactive menu.

Included with these changes:

• New command, "LaunchDarkly: Configure" which opens the configuration menu.
• When using the configuration menu, accessToken is stored in local storage.
• The sdkKey is now automatically inferred from the project and env configuration options. This makes sdkKey obsolete. So, if the SDK key happened to differ from the set project and environment, the behavior of the extension may change.
• The extension will now prompt users to configure itself on installation or update.
• The extension will prompt users to configure legacy configurations (where sdkKey is still set) until they do so or dismiss the notification.

[ldhenry]

Cool stuff here. Not really sure why someone would want to do this, but is it possible to use different API keys for different VSCode workspaces?

[atrakh]

@ldhenry-- yup! The last step of the workflow asks if you want to store the configuration for all workspaces, or just the current one. The storage type is used for both configurable user settings and internal extension storage.