feature #20612 [Filesystem] Add appendToFile() (chalasr)

This PR was merged into the 3.3-dev branch.

Discussion
----------

[Filesystem] Add appendToFile()

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | todo

So we could append content to a file:

```php
(new Filesystem)->appendToFile($file, 'bar');
```

instead of doing it in two steps:

```php
if (false === $content = @file_get_contents($file)) {
    throw new \Exception();
}

(new Filesystem)->dumpFile($file, $content.'bar');
```

Doing it opt-in using `dumpFile(..., $append = false)` would have been enough but not possible for BC.

Commits
-------

1f7b753670 [Filesystem] Add appendToFile()

src/Symfony/Component/Filesystem/CHANGELOG.md

@@ -1,6 +1,11 @@
 CHANGELOG
 =========
 
+3.3.0
+-----
+
+ * added `appendToFile()` to append contents to existing files
+
 3.2.0
 -----