Bismuth: fetch subcommand flag fixes

@ewanc26/bismuth v0.2.4

The fetch subcommand's parseArgs call was missing --frontmatter / -f and --output / -o, so passing either would throw an ERR_PARSE_ARGS_UNKNOWN_OPTION error. Both flags are now registered — --frontmatter takes precedence over --no-frontmatter if both are somehow passed, and --output / -o is an alias for --output-dir for consistency with the main convert mode.

Part of the @ewanc26/pkgs monorepo. Published to npm.