Improve slugify that isn't working with some cases

[aleaforny]

Let's take this example string : This IS- a C/O/MPLEX STRING - look@THAT

According to regular slugifiers (https://slugify.online/), this string slugified should be this-is-a-complex-string-lookthat

However, with default slugify(), it will returns a strange string with additional dashes, thus, not very slugify-friendly.

Here the output : this-is--a-complex-string---lookatthat

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With the codes changes, slugify() returns the expected string (I just also modified the remplacements.dart files to my convenience in order to prevent the '@' to become 'at', but I guess it depends on your system and your expectations)

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Explanations

• Somehow, the _dupeSpaceRegExp has been fixed.
• Then, the substition between the two regexes _dupeSpaceRegExp and _punctuationRegExp were to fix too; I changed the order. Now works as expected.