[Serializer] added support for is.* methods in GetSetMethodNormalizer

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10297
| License       | MIT
| Doc PR        | N/A

Using ``is`` prefix for boolean variables is considered a standard, not only in PHP but also in Java (from which Symfony2 derives lot standards).

I was not sure if this is BCB but answered "no". Was not sure if I should update ``CHANGELOG`` file and did so, but if you find it irrelevant, go ahead and merge without it. I don't know if I should create a PR for docs because it does not explicitly say that the normalizer supports only ``get.*`` methods as getters.

_Note: Objects that contain behaviour also can use other prefixes like ``can``, ``has``, ``should``, but their presence in ``GetSetMethodNormalizer`` is relevant as they do not provide state (``has`` is debatable)._