Description of the new feature/enhancement

Related to #1040, WT should set the COLORTERM environment variable to "truecolor" since WT does support truecolor. I can understand how changing TERM to "xterm-truecolor" (from "xterm-256color") could be a breaking change, but that's what COLORTERM is for.

Some tools like with cli/cli#4079 use this to determine if 24-bit colors should be emitted. Easy enough for users to define - either as Windows environment variables or update their shell profile e.g., .bashrc, but users who don't know what won't enjoy a wash of color by default like with gh issue list starting with v2.0.0.

Proposed technical implementation details (optional)

WT should ideally define the environment variable COLORTERM=truecolor within its own process environment block by default.

/cc @mislav