(PUP-2349) Add deprecation warning for non-string mode property values

The 'future' (4.x) parser treats literal numbers as actual numbers instead of
strings as the 3.x parser did. This makes file resources that use unquoted
mode values behave strangely (see PUP-2156) so we are deprecating non-String
values for mode.

So this change adds a deprecation warning for non-String values of mode
when running with --parser future.