Add config setting for changing "variant" keyword that is used as URL param in variant URL

[piotrpog]

This keyword is hardcoded in variant element class, here:

    public function getUrl(): string
    {
        return $this->product->url . '?variant=' . $this->id;
    }

[lukeholder]

Add a variantUrlParameterName column/field to Product Types table and product type editor page.

Description: The name of the Variant ID url parameter used when constructing a variant's URL

Default is variant. The Default variant URL would therefore be <productUrl>?variant=<variantId>.

Value would be inserted here: https://github.com/craftcms/commerce/blob/develop/src/elements/Variant.php#L564

[alexanderloewe]

1+ handle product slugs by commerce itself.