[11.x] Add existence check to PotentiallyTranslatedString class

[shaedrich]

Similar to $translator->has($key), you can now do $potentiallyTranslatedString->has()

[taylorotwell]

I am closing this pull request because it lacks sufficient explanation, tests, or both. It is difficult for us to merge pull requests without these things because the change may introduce breaking changes to the framework.

Feel free to re-submit your change with a thorough explanation of the feature and tests - integration tests are preferred over unit tests. Please include it's benefit to end users; the reasons it does not break any existing features; how it makes building web applications easier, etc.

Thanks!

[taylorotwell]

Need more info on what this allows end users to accomplish.