Add CLI completions to Bicep CLI

Some shells like PowerShell and bash support tab completion for various comments. Would be great to add this support to the Bicep CLI at some point.

.net uses a `complete` verb to achieve this (https://docs.microsoft.com/en-us/dotnet/core/tools/enable-tab-autocomplete), but there are also alternative options if Bicep CLI startup time becomes a bottleneck.

Start by reviewing the Bicep CLI command surface and the .NET `complete` verb referenced in the issue. Compare completion approaches for PowerShell and bash, including the stated startup-time concern. Done means the Bicep CLI provides usable tab completion in both shells.

Written by the indexing model from the issue text.