Add environment variable support in config.json

[itsneufox]

Implements ${VAR_NAME} and ${VAR_NAME:-default} syntax for environment variable expansion. Supports $$ escape sequence for literal $ characters.

Examples:

• Normal:

    "rcon": {
        "password": "${RCON_PASSWORD}"
    }

• With fallback default:

    "rcon": {
        "password": "${RCON_PASSWORD:-changeme1}"
    }

edit: this closes #1015