Add theme support for block template-parts

[crftwrk]

This PR allows to use blocks as template parts. In Backend Appearance > Template Parts can now the mini-cart edited in more detail and we can hard code it in the theme like woocommerce/storefront#2100.

Think in the long-term, our offcanvas-cart should be replaced by the mini-cart block #631.

Did roughly some changes to demonstrate how it works:

• Go to https://dev.bootscore.me/shop/ and add something to the cart
• Scroll down to the footer, there you will find the Woo mini-cart block. Open cart and check buttons, they have classes btn btn-primary etc.

@justinkruit IYLIMI

[justinkruit]

Personally, not the biggest fan of the block changes. This because it feels like giving over control to WooCommerce which has a bigger tendency to break stuff (see the alerts recently).

I'm also just seeing the block template parts system for the first time, and not entirely convinced. Will we be taking steps back from coding? Because Gutenberg has limited coding options in our current setup, which we can't really change unless we want to change how the theme works (making blocks with NPM compiling).

[crftwrk]

Will we be taking steps back from coding? Because Gutenberg has limited coding options in our current setup, which we can't really change unless we want to change how the theme works (making blocks with NPM compiling).

No, I do not mean to create a block theme, I like it as it is. But I think that the entire WP goes more and more to blocks and we have to find a way to be prepared. If for example Woo will remove the cart fragments entirely in future, we need an alternative for our mini-cart. This PR gives us the option to play around with it and maybe the mini-cart block can replace our one by hard coding it to the theme. This PR just allow us to edit blocks in more details and use them as template-parts, nothing more.

[justinkruit]

Sorry for taking so long, been busy.

Alright, I agree we can and should look into this. But do we want to enable this for this upcoming release? Or wait to expand on this on the release after that?

[crftwrk]

This has no effect to the theme but gives us more freedom to play around with blocks. Should be merged as soon as possible.

[justinkruit]

Alright, then lets add it to the theme and see where we go from there 👍