feat(accordion): add exportAs for brnAccordionItem and brnAccordion

[hillin]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our
  guidelines: https://github.com/goetzrobin/spartan/blob/main/CONTRIBUTING.md#-commit-message-guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

Which package are you modifying?

• accordion
• alert
• alert-dialog
• aspect-ratio
• avatar
• badge
• button
• calendar
• card
• checkbox
• collapsible
• combobox
• command
• context-menu
• data-table
• date-picker
• dialog
• dropdown-menu
• hover-card
• icon
• input
• label
• menubar
• navigation-menu
• pagination
• popover
• progress
• radio-group
• scroll-area
• select
• separator
• sheet
• skeleton
• slider
• sonner
• spinner
• switch
• table
• tabs
• textarea
• toast
• toggle
• tooltip
• typography

What is the current behavior?

Closes #535

What is the new behavior?

BrnAccordionDirective and BrnAccordionItemDirective can now be referenced by template variables with the added exportAs declaration.

e.g.

<div brnAccordion hlm #accordion="brnAccordion">
  <!-- items -->
</div>

{{ accordion.openItemIds() }}

Does this PR introduce a breaking change?

• Yes
• No

Other information

It's a simple change, I'm not sure if (and how) this should be tested.