Use :setting-predicate to assert the vars takes strings

Use :setting-predicate to assert the vars takes strings

In commit [cab32dcb1e], the variables `$newline`, `$tab`, and `$space`
were updated to assert that only strings could be assigned to them.

This is a slight change to use the `:setting-predicate` option to
`defmvar` instead of manually setting the `assign` property for these
variables.  I think this is easier to read and maintain.

Manually tested this to verify that we throw an error when setting
`$space` to a number.  Also verified that a string works as expected.