[Intl] Refactored Locale component into two new components Icu and Intl

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #5279
| License       | MIT
| Doc PR        | todo

The Intl component is now a simple drop-in replacement layer for the C intl extension. Install it via Composer and have it available automatically if the intl extension is not available.

Additionally, the component ships data from the ICU library which can be accessed through the methods:

```php
Intl::getCurrencyBundle()->...
Intl::getLanguageBundle()->...
Intl::getLocaleBundle()->...
Intl::getRegionBundle()->...
```

If the intl extension is installed, Composer will install the ICU data for the ICU version in the intl extension. If the intl extension is not installed, Composer will use stub ICU data for one of the latest ICU versions (see `Intl::getStubIcuVersion()`).

Todo:

* finish the README file
* create the following branches in the Icu component:
** 40-master
** 42-master
** 44-master
** 46-master
** 49-master
** 50-master
* update the documentation