[WIP] added back ICU classes from symfony/Icu

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #11447
| License       | MIT
| Doc PR        | n/a

The problem that this PR addresses is well explained in #11447 and the counterpart of this PR is the creation of the new `symfony/intl-data` repository, which is discussed in symfony/Icu#12. As explained there, there is a BC break as people will now need to explicitly add the `symfony/intl-data` dependency to get the ICU data.

my suggestion would be to add the requirement in 2.3 to 2.5 to avoid breaking existing apps, and then removing it from 2.6, asking users to require it explicitly when they upgrade to the new Symfony version.