Makes keyword arguments usage compatible w/ Ruby 3

[walski]

In Ruby 3 keyword arguments are completely decoupled from positional arguments (see e.g. ruby-lang.org News).

This fix makes the LinkFormatter compatible again with the new semantics of keyword arguments in Ruby 3.

[matthutchinson]

Looks good, and will work when calling Slack::Notifier::Util::LinkFormatter.format("some message"), but the initializer should probably be fixed too (since it's a public method).

Some people may be calling it like this;

Slack::Notifier::Util::LinkFormatter.new("some message").formatted

[6temes]

Is this gem still maintained? The last merge was in 2008.

[stevenosloan]

Just noting I'm merging this over #119 for the more descriptive commit message. Same change so I had to pick somehow 😅.

After adding some smoke tests around the usage of the initializer & formatted methods directly they don't appear to be affected by the keyword arg changes.