Add INI config file support

[jarz]

Add support for reading and writing INI configuration files using CLI11's built-in config infrastructure.

New options

• -c, --config <file> — Load settings from an INI file (defaults to dingusppc.ini in the working directory). The file is optional; if it doesn't exist, it's silently ignored.
• --write-config [file] — Export the current configuration (including any CLI overrides and machine properties) to an INI file and exit. Defaults to dingusppc.ini.

Example workflow

# Save a config with machine properties
dingusppc --machine pmg3dt --ramsize=128 --write-config

# Launch using saved config
dingusppc
# or explicitly:
dingusppc -c dingusppc.ini

Changes

• Enable config_extras_mode::capture so machine-specific properties (which are registered late) pass through to the config file and are available for lookup.
• Rewrite get_setting_value to look up properties from app.remaining() directly, handling --key=value, --key value (CLI), and bare key value pairs (config file).
• Mark --log-verbosity and --write-config as non-configurable so they don't round-trip into the INI file.
• Add dingusppc.ini to .gitignore.