Fix full stop error for '-singel'

[emacgillavry]

👋 I did some awesome work for the Pelias project and would love for everyone to have a look at it and provide feedback.

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Here's the reason for this change 🚀

• Fixes NL: recognise 'sngl' as abbr for 'singel' without full stop #165
• Adds test case

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Here's what actually got changed 👏

• add singel|sngl to concatenated_suffixes_separable.txt
• add example Blekerssngl, Gouda as a testcase

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Here's how others can test the changes 👀

• Without the change, both Blekerssingel, Gouda and Herensngl., Haarlem would pass, but Weerdsngl, Utrecht without a full stop indicating the abbreviation would not pass.
• With this PR, all the variations mentioned above do pass