[5.8] Fix JSON boolean queries

[staudenmeir]

Laravel 5.8's unquoting of JSON values on MySQL (#25732) broke boolean comparisons:

DB::table('users')->where('items->available', '=', true)->get();

// expected
select * from `users` where json_extract(`items`, '$."available"') = true

// actual
select * from `users` where json_unquote(json_extract(`items`, '$."available"')) = true

The json_unquote() call is actually only necessary for the SELECT clause, but I don't see a feasible/elegant way to compile different statements for the SELECT and the WHERE clause.

So I suggest we fix this by adding a new JsonBoolean type.
The default implementation works on SQLite 3.23+.

The is_string($column) check is necessary to preserve raw expressions:

->where(DB::raw("items->'$.available'"), '=', true)

I used the opportunity to also add support for PostgreSQL and SQL Server.

Fixes #27696.

[aaronhuisinga]

Thanks for the fix, @staudenmeir! Much appreciated.