Backport: Add Terms Query Pagination blocks

[jillro]

Introduce new blocks to enable pagination for terms queries, including "Previous", "Next", and "Page Numbers" links.

Adds build_terms_query_vars_from_block() and terms_query_register_query_vars() to wp-includes/blocks.php to support pagination in the Terms Query block.

These helper functions are used by:

• The updated Term Template block (which now supports paginated term results via the termspage query variable)
• The new Terms Query Pagination blocks (core/terms-query-pagination, core/terms-query-pagination-next, core/terms-query-pagination-previous, core/terms-query-pagination-numbers)

build_terms_query_vars_from_block() extracts and normalizes query arguments from the block context, handling both inherited (taxonomy archive) and custom term queries. It follows the same pattern as the existing build_query_vars_from_query_block() and build_comment_query_vars_from_block().

terms_query_register_query_vars() registers termspage as a public query variable so it can be used for pagination URLs.

The block files themselves (block.json, index.php, CSS, JS) are synced automatically via the Gutenberg build process.