Make keyword argument of arduino-cli core search optional

[per1234]

Current behavior

arduino-cli lib search lists all libraries in the index. However, arduino-cli core search does not mirror this behavior:

Error: requires at least 1 arg(s), only received 0
Usage:
  arduino-cli core search <keywords...> [flags]

Expected behavior

arduino-cli core search should be consistent with the arduino-cli lib search interface

Environment

• CLI version: built from source at 4d36bac
• OS and platform: Windows 10 64 bit

[howjmay]

I am the newcomer of this project.
May I take this issue?

[masci]

@howjmay sure thing, go for it!