Add forgotten is_basic_json guard to functions with json_pointer<Basi…

[alex-700]

…cJsonType> argument

• add regression test for regression: .value<size_t> is compilation error. #3655

Closes #3655

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• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
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Please don't

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[falbrechtskirchinger]

Unfortunately, this fix is not going to work. I've completely rewritten the value() functions. PR coming in a few hours.

There's also #3652, which requires a more substantial change.

[falbrechtskirchinger]

I should have looked a little closer. This does address a problem I did notice as well but isn't required to fix the regression after #3663.

[coveralls]

Coverage remained the same at 100.0% when pulling 85095fa on alex-700:latyshev/add-forgotten-is-basic-json into 2ac7d02 on nlohmann:develop.