dev: add typings with generic types to typst std types

[Myriad-Dreamin]

• add a new type-scheme analysis pass with support for typing metadata, recursive/self types, positional/named/variadic parameter shapes, and fresh type variables
• add snapshot-based tests for generated type schemas and for matching Typst standard-library type shapes
• introduce generated Typst typings for the standard library and update shared test fixture handling to support the new layout