[Yaml] Added support for object maps

Previously, the parser treated maps ( {} ) the same as sets ( [] ).
Both were returned as PHP associative arrays. Since
these are distinct entities, this can cause considerably problems for
the users, especially when YAML is being serialized into another
format such as JSON.

This commit allows the user to enable object-map support via a third
parameter on the Parse method.  It defaults to `false`, which means
that this commit does not break backwards compatibility.

If the user enables object-map support, maps are represented
by stdClass() objects.  Sets remain as arrays.
[Yaml] Cleanups for object maps
feature #10552  [YAML] Added support for object-maps (polyfractal, nicolas-grekas)

This PR was merged into the 2.5-dev branch.

Discussion
----------

 [YAML] Added support for object-maps

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | NA
| License       | MIT
| Doc PR        | NA

Proposal for https://github.com/symfony/symfony/pull/10114

Commits
-------

0cee604 [Yaml] Cleanups for object maps
e2d5468 [Yaml] Added support for object maps