ConPTY: environment-block support (post-2.0, if needed)

[darianmiller]

Post-2.0 follow-up. TConPty.Start currently inherits the parent environment.
Add an optional custom environment block if a use case needs it (parallel to the
legacy BuildEnvironmentBlock / NO_COLOR handling). Effort: S.

Acceptance criteria:

• An optional environment block is passed to the child when configured