Add the option to dump files to the ICU message format

[mark-gerarts]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	#550
License	Apache2

Description

As mentioned in #550: this commit adds the options to dump files in the ICU message format. This means the file gets suffixed with +intl-icu, e.g.: messages+intl-icu.en.yaml.

Some general remarks:

• The new options is called "intl-icu" (--intl-icu commandline, intl_icu in the config)
• The built-in translation interface works in normal situations, but behaviour is undefined you happen to use e.g. messages.nl.xliff as well as messages+intl-icu.nl.xliff. There is no indication which file you're editing, and no option to switch. It still works though, so I see this more as a thing for a future PR.
• As you can see in FileUtils: I took the liberty to use named capturing groups, because with another group added things were getting confusing.
• The classes I touched don't seem to have any relevant tests. I intend to add a test for FileUtils, but wanted to get this PR out first in order to get some feedback.
• I (manually) tested in a dummy project, but I intend to try this branch in a real-world project as well, either tomorrow or somewhere next week.

So, happy to hear your feedback!

[mark-gerarts]

Hmm, the CI fails because of an unrelated composer 2 issue?

Root composer.json requires doctrine/coding-standard ^7.0 -> satisfiable by doctrine/coding-standard[7.0.0, 7.0.1, 7.0.2].

[goetas]

thanks for your contribution, indeed we should use composer v1.x to avoid the CI failure