Make basic tests for accessing resources pass.
Overload DefaultFormatter in IcelandicFormatter, having to trim
plural endings that do not apply to Icelandic while anticipating
gender rules for Icelandic localisation of time based units with month
and day being masculine, year being neuter and others, eg. seconds
and hours taking the feminine gender. This called for access to
CultureInfo as base class does not expose this (improvement
suggestion?).
Register the Icelandic Formatter in Formatter Registry.
Some code branches cannot be unit tested in formatter as e.g.
Argument Exception can only be thrown if missing resource key.
Dual and TrialQuadral resource strings not included as they do not
apply to the Icelandic implementation.

Consisted of adding "og" to registry.

Similar tests as applied to e.g. German made to pass for Icelandic.
Using Tölvuorðasafn and Íðorðasafn as base for using kB for
kilobtytes and b as symbol for bit.

Relied on already translated resources and similar tests for other
languages of Germanic origin.

Included TimeOnly as usage in .NET 6.0 is approaching.

Icelandic has some complexity that can be attributed to being 'closer'
to its root than other surviving Germanic languages and having both
more irregularities and retaining inflections such as four case
synthetic grammar. This can mostly be represented with code
branching specific to this implementation. Among other thing the
first four natural numbers change based on gender and gender must
be applied to the last number in a sequence. Rules are mostly consistent for powers of ten, but 'hundred' varies between single and
plural. After performance measurements the final version was chosen
to closely align with other implementations, while trying to be as
transparent as possible.
The Icelandic rules for ordinals are have additional requirements, e.g.
on whether the penultimate number group or only the last match the
rule.
BenchmarkDotnet used on a parallel branch to make sure the
implementation compares favarobly to other localisations.

Code analysis run and no issues found in Icelandic language
support files.

Fix one mission EOL warning in PR view.

Restore AND handling code previously suggested by
code coverage to be superfluous, but that turned
out to be needed.

Added test and fix for the somewhat edge case of
"the one million and two thousandth" ordinal in Icelandic

Missing in _Above20 for seconds, days and years.

Forgot to add tests again after debugging