This PR was submitted for the 6.2 branch but it was merged into the 5.4 branch instead.

Discussion
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[String] CamelCase/SnakeCase on uppercase word

| Q             | A
| ------------- | ---
| Branch?       | 6.1
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | ~
| Tickets       | Fix #47421
| License       | MIT
| Doc PR        |

The behavior of `->snake()` method from String component has changed in 6.1.4, now, using `u('SYMFONY')->snake()` return `s_ymfony` instead of `symfony` (#47185).

After, some investigations, it seams the `->camel()` behavior is not exactly the expected one: `u('SYMFONY')->camel()` return `sYMFONY` instead of `SYMFONY`.

This PR give theses behaviors:
- CamelCase:
    - '' => ''
    - x_y => xY
    - xu_yo => xuYo
    - symfony_is_great => symfonyIsGreat
    - symfony_5_is_great => symfony5IsGreat
    - Symfony is great => symfonyIsGreat
    - Symfony is a great framework => symfonyIsAGreatFramework
    - \*Symfony\* is GREAT!! => symfonyIsGREAT
    - **SYMFONY => SYMFONY**

- SnakeCase:
    - '' => ''
    - x_y => x_y
    - X_Y => x_y
    - xu_yo => xu_yo
    - symfonyIsGreat => symfony_is_great
    - symfony5IsGreat => symfony5_is_great
    - symfony5isGreat => symfony5is_great
    - Symfony is great => symfony_is_great
    - symfonyIsAGreatFramework => symfony_is_a_great_framework
    - symfonyIsGREAT => symfony_is_great
    - symfonyIsREALLYGreat => symfony_is_really_great
    - **SYMFONY => symfony**

Commits
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c3cae1f [String] CamelCase/SnakeCase on uppercase word