config: Allow passing environment into environment variables

related: #669

assuming

session_name: django
environment:
  SERVER_PORT: "8012"
- window_name: test ${SERVER_PORT}
  layout: main-horizontal
  focus: true
  panes:
  - focus: True
  - blank

Allow window_name to receive SERVER_PORT without having to do env SERVER_PORT=8012 tmuxp load ./file.yaml

Step 1: Provide a summary of your problem

• For general technical questions, problems or feature requests related to the code in this repository file an issue.

Step 2: Provide tmuxp details

• Include output of tmuxp debug-info
• Include any other pertinant system info not captured

Step 3: Describe the problem:

Steps to reproduce:

Observed Results:

• What happened? This could be a description, log output, etc.

Expected Results:

• What did you expect to happen?

Relevant Code:

// TODO(you): paste here tmuxp configuration (YAML / JSON) you are having issues with.

Start by reproducing the YAML example from the issue and compare the current window_name expansion with the environment mapping. Trace the configuration-loading and variable-expansion entry points used by tmuxp, then add coverage for SERVER_PORT. Done means window_name receives the value declared under environment without requiring an external env command.