Add a generic is_numeric() function

[FFMG]

It would be nice to have a helper method is_numberic() similar to the php one.

It would be a virtual function in TJValue and all the TJComment/TJObject/TJArray/TJNull would return false, TJValueNumber would return true

For TJString we would have to check if it evaluates to a number ... if it could be a number

Not sure what we would do about TJBoolean

Maybe we should have a 'strict' flag, so that TJBoolean and TJString return false if strict?

[useroutofbound]

I have fixed the build issues by adding the override specifiers to the
is_numeric declarations in all derived classes within src/TinyJSON.h.

[useroutofbound]

Added the test file too, please check and confirm