Infrastructure fixes

- [x] specification `Immutable` structure named `Mutable` - [x] Use new [markdown parser](https://github.com/kaleidawave/simple-markdown-parser) in specification tests - [x] Use markdown parser in code blocks to script - [x] unflag and check tree-shaking test - [x] more things should be behind the number intrinsics flag - [x] https://github.com/kaleidawave/ezno/discussions/222 - [x] Redo NaN implementation while fixing #224 - [x] Fix mapped property test - [x] Fix less-than equal narrowing - Turns out reordering was actually turning `2 = a` into `a = a` - [x] Improve performance script