fix(ngList): use custom separators for re-joining list items

The separator string used to split the view value into a list for the model
value is now used to join the list items back together again for the view value.

BREAKING CHANGE:

The `ngList` directive no longer supports splitting the view value
via a regular expression. We need to be able to re-join list items back
together and doing this when you can split with regular expressions can
lead to inconsistent behaviour and would be much more complex to support.

If your application relies upon ngList splitting with a regular expression
then you should either try to convert the separator to a simple string or
you can implement your own version of this directive for you application.

Closes #4008
Closes #2561
Closes #4344