insert_final_newline should ensure a single newline character at the end of the file

[matrss]

Right now, editorconfig-checker does not complain about multiple newlines at EOF when the insert_final_newline option is set.

According to this comment on a feature request to support specifying the number of newlines that should be at EOF the option is intended to mean that a file should end in a singular newline, so editorconfig-checker should complain about multiple newlines at the end of a file.