[mono-api-tools] Split properly for cross-platform (#9918)
When creating a diff on windows, the lines are split on the `\n` so the `\r\n` results in multiple lines:
**START SNIP**
Obsoleted methods:
```diff
[Obsolete ("Use AddRoundRect instead.")]
public void AddRoundedRect (SKRect rect, float rx, float ry, SKPathDirection dir);
```
**END SNIP**
This PR makes sure to use the platform line endings everywhere. When adding line endings, use the same that `StringBuilder` uses - platform endings. This makes it easy to split - just use the platform as that is what everyone is using.