Support a configurable display timezone for comment timestamps

[ItsMalikJones]

Closes #17.

Summary

Comment timestamps are stored in the app's default timezone and, until now, were
always rendered in that same timezone. This PR adds an opt-in way to render them
in a different timezone — either a fixed app-wide value or a per-request value such
as the authenticated user's timezone.

What changed

• New config key commentions.timezone (defaults to null).
• New Config API:
  • Config::resolveTimezoneUsing(?Closure $callback = null) — register a closure
    for per-request resolution; passing null clears it.
  • Config::getTimezone(): ?string — resolves the effective timezone.
  • Config::applyTimezone(DateTime|CarbonInterface $dt) — returns a timezone-adjusted
    copy of a date.
• Comment::getCreatedAt() / getUpdatedAt() and the matching RenderableComment
  getters now pass their value through Config::applyTimezone().
• Documentation added to README.md and config/commentions.php.

Behaviour details

• Resolution order: the closure result takes precedence; when it returns null
  it falls back to the commentions.timezone config value; when neither yields a
  value, dates render in the storage timezone (unchanged behaviour).
• Non-mutating: applyTimezone() returns a copy (->copy() for Carbon,
  clone for DateTime) and never mutates the source instance.
• Blank-safe: an empty-string result (e.g. an unset users.timezone column) is
  treated the same as null, so it can't throw an InvalidTimeZoneException at
  render time.

Usage

// config/commentions.php — fixed, app-wide
'timezone' => 'America/Chicago',

// per-user resolution
use Kirschbaum\Commentions\Config;

Config::resolveTimezoneUsing(fn () => auth()->user()?->timezone);

Testing

Added tests/TimezoneTest.php (7 tests) covering: no-timezone passthrough, fixed
config conversion, the RenderableComment path, closure-over-config precedence,
config fallback when the closure returns null, empty-string handling, and
source-instance non-mutation.