[Core] Implement export VAR=value and Variable Expansion

[aphexlog]

Enable setting and expanding environment variables in ZorpSh.

Tasks:

• Parse export VAR=value and store the variable.
• Allow expansion of $VAR in commands.
• Ensure echo $VAR prints the expected value.

Acceptance Criteria:

• Running export TEST=hello sets the variable.
• Running echo $TEST prints hello.
• Running env lists all environment variables.