Keep explicit newlines with help texts

This makes a difference for e.g. pytest-xdist:

Before:
```
  --dist=distmode       set mode for distributing tests to exec environments. each: …
                        available environment. loadscope: …
                        grouped by file to any available environment. (default) no: …
```

After:
```
  --dist=distmode       set mode for distributing tests to exec environments.
                        each: send each test to all available environments.
                        load: load balance by sending any pending test to any available environment.
                        …
                        (default) no: run tests inprocess, don't distribute.
```

This might also result in unexpected changes (hard wrapping), when line
endings where used unintentionally, e.g. with:

```
help="""
    some long
    help text
    """
```

But the benefits from that are worth it, and it is easy to fix, as will
be done for the internal `assertmode` option.

src/_pytest/config/argparsing.py

@@ -509,3 +509,15 @@ class DropShorterLongHelpFormatter(argparse.HelpFormatter):
         formatted_action_invocation = ", ".join(return_list)
         action._formatted_action_invocation = formatted_action_invocation  # type: ignore
         return formatted_action_invocation
+
+    def _split_lines(self, text, width):
+        """Wrap lines after splitting on original newlines.
+
+        This allows to have explicit line breaks in the help text.
+        """
+        import textwrap
+
+        lines = []
+        for line in text.splitlines():
+            lines.extend(textwrap.wrap(line.strip(), width))
+        return lines