BREAKING(cli/unstable): change custom formatter rule

[timreichen]

This PR removes the default addition of the [] brackets from formatter elements. This makes it easier to use a custom format without string slicing.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.70%. Comparing base (ba1ee68) to head (350dccf).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6678   +/-   ##
=======================================
  Coverage   94.70%   94.70%           
=======================================
  Files         563      563           
  Lines       46676    46676           
  Branches     6573     6573           
=======================================
  Hits        44204    44204           
  Misses       2430     2430           
  Partials       42       42           

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[kt3k]

Can you also update Custom Formatting example in jsdoc of the class?

[kt3k]

This allows users to use other kinds of parens/brackes. This change makes sense to me.

[kt3k]

LGTM

[kt3k]

The next @std/cli release will already include 3 breaking changes to ProgressBar #6406 #6409 #6430

Let's make this change as well at once instead of making breaking changes separately in multiple releases