Add string in map binary operator to core extension

[veewee]

This PR makes it possible to validate if a string key exists in a map.

'field' in answers

Results in truefor:

[
    'answers' => [
        'field' => 'available',
    ],
]

Results in falsefor:

[
    'answers' => [
    ],
]

https://playcel.undistro.io/?content=H4sIAAAAAAAAA3WSTUsDQQyG%2F0pcD6tQ23tvRQoqVYsiRegl3U23AzOTJTPbD8T%2FbrIulB7KXEIyed43mfkpKvLFtJhMYEW%2B4kCQGfKO4HG%2BgKXHUyPcxfpmHfXKZQ5cAozgYibBKrs9KWMzS4nCxp%2Bg5QMJ1UBx74RjoJiNTcfWs5B21haTuL5ycHn3r8shcIT5sRVKyWm4wNh02BDcqf792Jz0bh5gJS4TnLiT3hqde3YqPVz6StSDUQhBSxYnVxNsWdR822WoMeNIY%2FievS5A0y%2Bf729WD5gHytLIUH50seyn2KPv8Io4Nuhiyr3SWWAAzYf5ra9i70k3pz28VQKG1lMajKXWCVptrafcMpfmEKtKd6%2BuilFh1Gfj6wPewpOObE9yqXplqrFBB9Z0HUHTPIUNimID16RE%2Bxi%2Ff4mcxtggAgAA

[coveralls]

Pull Request Test Coverage Report for Build 19233036954

Details

• 4 of 4 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 71.931%

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Totals
Change from base Build 19236417288:	0.03%
Covered Lines:	3211
Relevant Lines:	4464

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💛 - Coveralls