[6.x] Fix mysql schema for srid (point)

[rodrigopedra]

As described in #31543 if a migration add a column of type POINT using the SRID parameter and also uses the after modifier, the generated SQL is invalid.

Steps to reproduce:

Schema::create('test', function (Blueprint $table) {
    $table->bigIncrements('id');
});
        
Schema::table('test', function (Blueprint $table) {
    $table->point('location', 4326)->after('id');
});

Expected SQL:

alter table `test` add `location` point not null srid 4326 after `id`;

Actual SQL:

alter table `test` add `location` point not null after `id` srid 4326;

When adding modifiers, the Grammar base class iterates over the $modifiers array and processes them in other order:

https://github.com/laravel/framework/blob/6.x/src/Illuminate/Database/Schema/Grammars/Grammar.php#L144-L161

This PR, moves the 'Srid' modifier sooner in the MySqlGrammar's $modifier array, so it is processed before the relevant modifiers.

I added two tests:

• one to check syntax using the SRID without the after modifier
• one to check syntax using the SRID with the after modifier

This a submission of PR #31851 to the 6.x branch

[halloei]

Thanks @rodrigopedra !