feat(typlite): render context block contextually

[hongjr03]

• Better contexual render

  I introduced an EcoString to store the codes of nodes that result in Value::None (excluding Ident | Bool | Int | Float | Numeric | Str | Array | Dict and Hash at the beginning of #context) that appear before a Contextual node. Then prepend the buffer to the Contextual node to render in right environment.

  Add assets_path and assets_src_path to args and TypliteFeat, support render contexual code to external svg file.

Variable names were chosen quickly and are not ideal apparently, sorry for that :(

[Myriad-Dreamin]

The code here has bad performance. Assuming there are $O(N)$ statements in your document and you render typst code many times, the prepend_code will copy the content in $O(N^2 \log N)$ of time complexity.