Formatting strike prices for accessibility; improvement to WC pricing to use new is_type function

[kimcoleman]

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Changes proposed in this Pull Request:

Accessibility changes to how we show strikethrough pricing in Easy Digital Downloads module using recommends from accessibility expert Amber Hinds.

Note also updates to use the is_type function in WooCommerce to check product type before showing strikethrough pricing. We are not including subscription products at this time - in the future if we do include these here is a reference on product types for subscriptions: https://www.businessbloomer.com/woocommerce-check-if-product-is-subscription/.

$product->is_type( 'subscription' ) and $product->is_type( 'variable-subscription' )

Resolves #84 Resolves #73

Other information:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [x] Have you successfully run tests with your changes locally?

Changelog entry

• ENHANCEMENT: Improved accessibility of strikethrough pricing for EDD module.
• ENHANCEMENT: Improved display of strikethrough pricing to match core WooCommerce.
• ENHANCEMENT: Now using the WooCommerce is_type function to check if product is single or variable.

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.