Infrastructure fixes

[kaleidawave]

• fix issue with unescaped HTML in comparison document
• fix playground
• specification Immutable structure named Mutable
• Use new markdown parser in specification tests

TODO

• Use markdown parser in code blocks to script
• Use markdown parser via WASM to build specification page
• Make a page for https://kaleidawave.github.io/ezno/
• unflag and check tree-shaking test
• add build command to playground
• more things should be behind the number intrinsics flag
• simplify playground build into one script
• `wasm-bindgen` version conflict resolution? #222