(#22363) Add test for interpolation string conversion undef / default

There was no test for evaluation result of undef or default being
interpolated.
(#22363) Add more String interpolation tests
(#22363) Remove ablity to apply operator % to floats

The result is almost always surprising when trying modulo operator
on floats. It only works in a meaningful way if the floating point
numbers are integral in the first place. Rounding errors makes results
very confusing. An error is now issued instead that % is not applicable
to floats.
(#22363) Add test that shows negaice shift reverses direction

When shifting a negative count the direction of the shift is changed.
This adds tests that shows this.
(#22363) Add test that false != ''

A falsey value is not equal to false.
(#22363) Remove support for compound assignments

Compund (deflating) assignments where not completely implemented
and it turned out to be a bad idea (i.e. indirect variable
creating where [a,b,c] = [1,2,3] produces a = 1, b = 2, c = 3,
and [a,b,c] = { x=>10, a=>1, b=>2, c =>3} produces the same).

Not much is saved in terms of typing while having poor readability.
It also adds further things that can not be statically analyzed
since variable names can be dynamically computed. This can be abused.

The construct was removed instead of completed.

This also adds assignment tests for =, += and -=