[8.x] Fixed return type of Grammar::getValue

[robotsniper]

this return type should be consistent with Expression class getValue method return type

[sebdesign]

I would suggest that we revert this change.

The actual return type of this method depends on the return type of Expression::getValue().
While this method returns mixed, this is something that was added in the first commit of Laravel!
See: https://github.com/laravel/framework/blame/599e7851988ce88aae745847389dcefd3c4cc6f6/src/Illuminate/Database/Query/Expression.php#L26

The Expression also has a __toString() method that casts the result of getValue() to string, which is risky, because if the value cannot be casted to string, that would raise an error.

Also the result of Grammar::getValue() is used in many methods that return string, like wrapTable(), wrap(), parameter().

The most sane thing to do is to change all the references of mixed types to \Stringable|scalar in the Expression class in 9.x as it is a breaking change.