added optgroup form examples

[kevinpapst]

@rjd22 here is a PR that adds optgroup examples in the form page, so they can be tested easily

See also #1024 and #1098

This is default browser (Safari) rendering:

And this is how Tom-Select currently looks:

This is how it looks normally with TomSelect and Bootstrap 5: https://tom-select.js.org/examples/optgroups/

UPDATE
I found the issue that is causing it.
You are importing the tom select bootstrap 5 css file, but you are preventing the rules from being applied. They are prefixed like this: .#{$select-ns}-dropdown where select-ns variable is ts by default, resulting in rules like these:

.ts-dropdown {
	.optgroup-header {
		font-size: $font-size-sm;
		line-height: $line-height-base;
	}
}

But the classes are not applied in the select.html include.
Is that on purpose @codecalm? By re-applying them (I did in the PR) the dropdown now looks better:

Now dark mode still has an issue, it shows the header with white background. This rule makes it slightly better:

.ts-wrapper{
  .dropdown-menu {
    .optgroup-header {
      color: var(--#{$variable-prefix}body-color);
      background-color: var(--#{$variable-prefix}body-bg);
    }
  }
}

but I am not entirely happy with the result and would like to leave this up to you, because you simply know better.

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[kevinpapst]

by adding this class, the original tomselect rules now will be applied

[rjd22]

LGTM

[kevinpapst]

Ha, I already forgot about this PR 😁

Thanks @rjd22 for merging 👍