Add support for .permissions.yml files

This became WAY easier thanks to #2322839: Unify YAML translation extraction, and allow extension by contrib.. We can postpone this on #2295469: Add support for static permission definitions with *.permissions.yml.

All right, #2295469: Add support for static permission definitions with *.permissions.yml just landed.

#2322839: Unify YAML translation extraction, and allow extension by contrib. makes this theoretically easier to do but then it does not support a direct value extraction from an arbitrary key if its not an array. So this will still fail. We either need to make up some sensible syntax for the 'not an array, take the key' variant or just go back to the manual parsing solution for this file. Will summon @herom for this question :)

Sorry no interdiff, but I used the test yaml file and the test PHP code verbatim from @dawehner.

Rolled a variant of @dawehner's patch instead.

Helps to check the file name with the good variable.

I think this might pop up in other places. So, let's try handling this the NEW way.
I have used the empty string ('') as special key, to mark all top-level keys as translatable. This won't be recursive, and will leave array values alone.

Edit:
It will look like this

+++ b/yaml_translation_patterns.yml
@@ -22,5 +22,9 @@
+*.permissions.yml:
+  - title
+  - description
+  - ''

Updated to use '%top-level-key', after discussing with @Gabor.

Made some minor whitespace fixes and committed. Thanks a lot!

BTW I've been pondering a bit around %top-level-key vs. %top-level-value, because you actually make the value translatable, but in all other cases, you put in the key name, so referring to the key here makes sense for consistency. Also for those not aware this was suggested for 'consistency' with %key, %value, %parent, etc. in config schema.

The porting will be done in #2322839: Unify YAML translation extraction, and allow extension by contrib..