[9.x] Support modifying a char column type

[hafezdivandari]

This PR provides support for modifying a char column type.

Schema::table('users', function (Blueprint $table) {
    $table->char('name', 50)->nullable()->change();
});

Before this PR, modifying a char column led to an error:

Unknown column type \"char\" requested. Any Doctrine type that you use has to be registered with \\Doctrine\\DBAL\\Types\\Type::addType(). You can get a list of all the known types with \\Doctrine\\DBAL\\Types\\Type::getTypesMap(). If this error occurs during database introspection then you might have forgotten to register all database types for a Doctrine Type. Use AbstractPlatform#registerDoctrineTypeMapping() or have your custom types implement Type#getMappedDatabaseTypes(). If the type name is empty you might have a problem with the cache or forgot some mapping information

But doctrine/dbal package actually supports modifying char column types as StringType::class by setting fixed option to true.

So this PR, maps Laravel char to its Doctrine equivalent string type and set fixed option to true that finally gets the SQL snippet to declare a CHAR column.

[taylorotwell]

Can you describe how you determined these changes were needed and what the problem was before this fix?

[taylorotwell]

Please mark as ready once you have done so.

[hafezdivandari]

@taylorotwell sure, I updated my comment above.