Add merge subcommand to sing-box rule-set

[surinrasu]

Merge multiple rule-set json files

Usage:
  sing-box rule-set merge [flags] <source-path-1> <source-path-2> [<source-path-3> ...]

Flags:
  -h, --help           help for merge
  -w, --write string   write result to a file instead of stdout

Global Flags:
  -c, --config stringArray             set configuration file path
  -C, --config-directory stringArray   set configuration directory path
  -D, --directory string               set working directory
      --disable-color                  disable color output

When I was creating my own rule set, I realised I needed a tool like this, so I wrote this subcommand
I'm not expert of Golang nor familiar with this repo, so I'm sorry and please feel free to modify it if there's anything should be improved 🥺

[nekohasekai]

Try f85a7e8

[surinrasu]

Try f85a7e8

Thanks for your work and review! It works perfectly well🥰