Changelog script. Add ability to check PRs (#5203)

Needed for
https://youtrack.jetbrains.com/issue/CMP-7409/Add-a-CI-check-for-PR-containing-Release-Notes

Also fix a bug with not working N/A release notes

## Testing
```
action=checkPr JetBrains/compose-multiplatform 5188
```
works with different values correctly

```
changelog.main.kts v1.8.0-alpha01..HEAD 
```
excludes N/A

Better to compare not via GitHub diff, as it hard to compare identated
code.

## Release Notes
N/A