SI-7969 REPL -C columnar output

Let `ConsoleReaderHelper.printColumns` print in a vertical
orientation (similar to `ls -C`) instead of horizontally (`ls -x`).

To support the old behavior, add a property `-Dscala.repl.format=across`
where "across" is the presumptive mnemonic for `-x`.

The format property also takes value `paged` to turn on pagination,
which currently forces single-column output (as does `ls | more`)
for simplicity.
SI-7969 Refactor to trait with test

Make REPL classes testable in junit. Test the Tabulator.
SI-7969 REPL variable columnar output

Extend column formatting to make columns only as wide as
their widest element.  This is similar to what `ls` does.

Given the longest and shortest string, which bound the
min and max column count, compute the layout for those
possible column counts, and choose the minimal row count
and minimal column count.

The junit test is under pending because it uses expecty.
(Edit: updated without expectytations.)

Example that really benefits, witness the skinny columns:

```
scala> math.

BigDecimal                     PartiallyOrdered             cosh      rint
BigInt                         Pi                           exp       round
E                              ScalaNumber                  expm1     signum
Equiv                          ScalaNumericAnyConversions   floor     sin
Fractional                     ScalaNumericConversions      hypot     sinh
IEEEremainder                  abs                          log       sqrt
Integral                       acos                         log10     tan
LowPriorityEquiv               asin                         log1p     tanh
LowPriorityOrderingImplicits   atan                         max       toDegrees
Numeric                        atan2                        min       toRadians
Ordered                        cbrt                         package   ulp
Ordering                       ceil                         pow
PartialOrdering                cos                          random

```

more
Merge pull request #3147 from som-snytt/issue/repl-columns

SI-7969 REPL -C columnar output