Blocks/refactor twentytwentyone block processor (take 2)

[dmsnell]

Trac ticket: Core-64482 See: Core-61401 See: ~#9105~, ~#9841~, (~#9842~ -> #10291), ~#9843~ -> #10292

Refactors the excerpt-generating functionality in twentytwentyone to use WP_Block_Processor and avoid loading entire posts into memory. This streaming approach scans through the post contents and only parses as much as is required to provide the requested blocks.