Add `item_is_table` for block items #2294

Job	Run time
Clippy	33s
Documentation	26s
Build Benchmarks (w/ yoga)	55s
Build Benchmarks	45s
Test Fixture Formatting	14s
Rustfmt	14s
Markdown Lint	4s
Test Suite [alloc + all algorithms]	1m 36s
Test Suite [std + block)]	1m 37s
Build [debug feature]	16s
Test Suite [default features]	1m 33s
Test Suite [default except content_size]	1m 39s
Test Suite [default + serde]	1m 43s
Test Suite [std + flexbox]	1m 37s
Test Suite [std + grid]	1m 42s
Test Suite [alloc + no algorithms]	1m 46s
MSRV Build [Rust 1.65]	2m 2s
Test Suite [std (no algorithms)]	1m 39s
Test Suite [no features]	1m 38s
	21m 59s

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