Automatic ASCII content population for branches in .tree files

[scorpion9979]

Currently, creating .tree files as per the BTT framework involves either manually transferring the ASCII content of the branches, or to trust GitHub Copilot to accurately predict what ASCII symbols to fill in after already having written several lines in the .tree file.
An improvement for the dev experience would be to implement an auto-complete feature for tree branches that automatically fills in the ASCII content of a branch as a developer begins a new line. Furthermore, this feature would dynamically modify the ASCII content based on changes in line indentation.

[PaulRBerg]

@scorpion9979 you might find the Ascii Tree Generator](https://marketplace.visualstudio.com/items?itemName=aprilandjan.ascii-tree-generator) extension helpful.