Make types of parameter of join method consistent in the Query Builder

[melicerte]

There is a consistency problem with the typing of the $second parameter in functions that perform joins.

In the JoinClause class, the "on" function accepts an Expression in the $second parameter:
@param \Illuminate\Contracts\Database\Query\Expression|string|null $second

Which is perfectly normal.

However, in the "join" function of the "Builder" class, the "JoinClause::on" function is called with the $second parameter, which is not typed "Expression".

At runtime, you can pass an expression in $second, which poses no problem and the request is executed correctly.
However, if you use a static analysis tool like phpstan, the compatibility problem appears:

Parameter #4 $second of method Illuminate\Database\Query\Builder::leftJoin() expects string|null,
         Illuminate\Contracts\Database\Query\Expression given.

My suggestion is to add the "Expression" type to the $second parameters in order to guarantee type consistency in the Query Builder at this level.