feat: add defaults for the import command's --input-path and --repository-mappings-path arguments

[joshjohanning]

For importing, setting defaults for the --input-path and --repository-mappings-path parameters.

I would imagine most people running this are going to run the export in a folder, update the repo mappings, and then run the import. Setting defaults saves some typing/copy+pasting 😄

And also, the gh migrate-project --help already mentioned defaults (I realize it was mentioning the default for the export command, but we might as well default it for the import command too.

--input-path <input_path> The path to the exported project data. This will be the --project-output-path argument passed to the export command, which defaults to project.json.
--repository-mappings-path <repository_mappings_path> The path to your completed repository mappings file. This will be the --repository-mappings-output-path argument passed to the export command, which defaults to repository-mappings.csv.

[timrogers]

😍 This is an obviously sensible change, and something I should have done! Thanks for the suggestion.