Omit the trailing newline with string gaps

See https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/multiline_strings.html#examples:

Expression
```
"""
Line 1
Line 2
Line 3\
\"""
```

Output
```
   "Line 1\n"
++ "Line 2\n"
++ "Line 3"
```