Add a walkthrough experience for the VSCode extension

[bwateratmsft]

Closes #1404

Install step. This step will be hidden ~1 second after extension activation if the CLI is already installed. Step is marked completed when they install, or automatically if it was already installed before opening.

Sign in step. Similarly, will be hidden ~1 second after activation if already logged in. Step is marked completed when they sign in, or automatically if they were already signed in when they open it.

Scaffold step. Similarly, will be hidden ~1 second after activation if an azure.yaml (or azure.yml) file exists at the root of the workspace--scoped as such for performance. Step is marked completed when they init, or if project was scaffolded before opening.

Up step. Step is marked completed when the user does up, provision, deploy, or infra-create; though the button specifically does up. The "Command Palette" link will pre-type "> Azure Developer: up" to immediately show the relevant command.

Learn More step. Step is marked completed when it is clicked on to. The "Command Palette" link will pre-type "> Azure Developer: " to immediately show the relevant commands.

Light theme of Learn More step (scaffold and up steps work the same way)

[savannahostrowski]

A couple of copy nits but other than that, LGTM.

[azure-sdk]

VSCode Extension Installation Instructions

1. Download the extension at https://azuresdkreleasepreview.blob.core.windows.net/azd/vscode/pr/1644/azure-dev-0.4.0-alpha.1.vsix
2. Extract the extension from the compressed file
3. In vscode
   a. Open "Extensions" (Ctrl+Shift+X)
   b. Click the ...\ menu at top of Extensions sidebar
   c. Click "Install from VSIX"
   d. Select location of downloaded file