create new files from vscode's "Create: New File..."

## Problem

Toolkit has various "create new file" commands:

- `AWS: Create new CloudFormation Template`
- `AWS: Create new SAM Template`

## Expected behavior

Hook into vscode's `Create: New File...` command instead of implementing bespoke commands.

Start by locating the implementations and registrations for the AWS: Create new CloudFormation Template and AWS: Create new SAM Template commands in the extension. Read how those commands create files, then trace the VS Code Create: New File... integration point. Done means the existing template choices are available through that command without relying on bespoke commands.

Written by the indexing model from the issue text.