[Translator] Add comments when dumping po files

[deguif]

Q	A
Branch?	master
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	?
Fixed tickets	#29962
License	MIT
Doc PR

This code

$catalogue = new MessageCatalogue('fr');
$dumper = new PoFileDumper();

$catalogue->set('key.one', 'First key', 'custom');
$catalogue->setMetadata('key.one', ['sources' => 'src/file_1', 'comments' => 'Comment', 'flags' => 'fuzzy'], 'custom');

$catalogue->set('key.second', 'Second key', 'custom');
$catalogue->setMetadata('key.second', ['sources' => ['src/file_1', 'src/file_2'], 'comments' => ['Comment 1', 'Comment 2'], 'flags' => ['fuzzy', 'another']], 'custom');

$dumper->dump($catalogue, [
    'path' => 'xxx',
]);

Will produces this output:

msgid ""
msgstr ""
"Content-Type: text/plain; charset=UTF-8\n"
"Content-Transfer-Encoding: 8bit\n"
"Language: fr\n"

# Comment
#, fuzzy
#: src/file_1
msgid "key.one"
msgstr "First key"

# Comment 1
# Comment 2
#, fuzzy,another
#: src/file_1 src/file_2
msgid "key.second"
msgstr "Second key"

[Nyholm]

I like this PR. Thank you
👍

---

Just and FYI:

In xliff we have notes and target-attributes. Here we introduce sources, comments and flags.

We maybe should have 4 standard thing keys that are shared over all dumpers/loaders.

[fabpot]

Looks like the tests are broken by the features introduced in this PR. @deguif Can you have a look?

[deguif]

Please don't merge, I will rebase it once #30919 will get merged.

[fabpot]

@deguif You can rebase this one now.

[deguif]

@fabpot this should be mergeable, test failures are not related.

[fabpot]

Thank you @deguif.