[Command change] - check-unused-l10n - add possibility to exclude keys by mask

[furaiev]

It would be nice to have a possibility to exclude some keys by mask:
E.g. flutter pub run dart_code_metrics:metrics check-unused-l10n lib -p ^S$ --exclude 'mcc*|any_other_mask'

  "hello": "Hello",                             <<< doesn't excluded
  "@mcc3299": {
    "type": "text",
    "placeholders": {}
  },
  "mcc3299": "Airlines",                 <<< excluded (mcc*)
  "@mcc3299": {
    "type": "text",
    "placeholders": {}
  },
  "mcc3351": "Car Rental",              <<< excluded (mcc*)
  "@mcc3351": {
    "type": "text",
    "placeholders": {}
  },

Why?
Some localisations aren't used in code directly but come from the server.