string: split on whitespace by default.

* specs/string_spec.yaml (split): Add an example with no explicit
split-pattern argument.
* lib/std/base.lua (split): Default split-pattern to `%s+` when
no argument provided.
* lib/std/string.lua (split): LDocs cite `%s+` as the default
pattern instead of `%s*`.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>