Generate only 1, not 2 newlines at the end of baseline

[VasekPurchart]

We have noticed, that baseline has two newlines at the end of file, which is weird because the convention for most text based files is to have one. I believe PHPStan follows it as well, even for neon, e.g.: https://github.com/phpstan/phpstan-src/blob/831469c9c406f18e537dd8c46b8b9b28a3897450/build/phpstan.neon .

But baseline is generated using Neon::encode() and this puts two newlines at the end of file. From my research I believe this is due to the fact that the encoding puts empty lines between array items and there is no exception for the last one.

I do not believe that changing this in Neon is desirable - too big of a change for too little gain. Plus if it were to be omitted only after the last item, it could lead to uneven results with concatenating multiple separately encoded neons etc. On the other hand in PHPStan we already know the structure of the whole file, so we can solve it here easily.

I included measures not to change the baseline file if this would be the only change in order to prevent bugging people with it. It will disappear with the next "real" change.

[ondrejmirtes]

I took this step so that this change can be properly testable: 3a3c69e

Wer're now free to change the formatErrors() method signature to pass in an existing baseline file.

The desired behaviour can now be nicely tested in BaselineNeonErrorFormatterTest.

[ondrejmirtes]

Thank you!