Request: "Tab title" text resolves environment variables

# Description of the new feature/enhancement
When entering the "Tab title" text in the settings, it would be great if environment variables could be resolved. When I've tried this, for example with standard Command Prompt, all I get are strings of "%SOMEENVIRONMENTVAR%" in my tab title.

# Proposed technical implementation details (optional)
For all "Tab title" strings, just run an expand pass that attempts to substitute any "%..%" sub-strings with any available environment variable.

Start by locating the Windows Terminal settings implementation for the “Tab title” field and tracing how its text is currently applied. Check whether environment-variable expansion is already handled elsewhere, then verify that available variables written as %NAME% resolve in tab titles without changing ordinary text.

Written by the indexing model from the issue text.