docs: add variable expansion documentation to README

[Copilot]

The README had no documentation for the ${VARIABLE_NAME} variable expansion feature supported by ts-appconfig.

Added a Variable Expansion section to README.md covering:

• Basic expansion — referencing other variables via ${VAR} in .env or schema defaults
• Nested expansion — multi-pass resolution (variables referencing variables that are themselves references)
• Number references — number-typed variables are cast to string on expansion
• Error cases — CircularReferenceError (mutual references) and UndefinedReferenceError (unknown variable name), both thrown at configuration time

Example:

APP_HOST=localhost
APP_PORT=3000
APP_URL=http://${APP_HOST}:${APP_PORT}
# env.APP_URL → "http://localhost:3000"