Create unique case insensitive column for MySQL and SQLite

Question

I am using Laravel 8 and I have this table:

Schema::create('users', function (Blueprint $table) {
    $table->id();
    $table->string('username')->nullable()->unique();
});

In MySQL the username is caseinsensitive unique. This means I cannot have two rows like this:

id | username
1  | MAX
2  | max

However, in SQLite, it its casesensitive unique, meaning the above example would be possible.

I found at https://stackoverflow.com/a/20914992 how the raw SQLITE command is for this. However, I don't want to use a raw SQL command, because I would like that my migrations work for both, SQLite and MySQL.

I found a similar question at Laravel Schema Builder | Unique, Case Sensitive Column which asks for the opposite and suggests to use the collation migration-modifier to set to an case-sensitive collocation. Unfortunately, the collation does not seem to work for SQLite on Laravel but only for (MySQL/PostgreSQL/SQL Server).

How can I adjust the migration, so that the column is unqiue case insensitive for MySQL and SQLite?