Source position information in AST

[dorchard]

Currently the fluid AST does not contain any information about source position as far as I can see. I think it would be very useful to add this (e.g., start and end positions per AST node) to aid in user-facing tools, like type checkers and more fine-grained provenance information in the future. It looks like this would be fairly straightforward to add leveraging the combinators in purescript-parsing (e.g., https://pursuit.purescript.org/packages/purescript-parsing/10.2.0/docs/Parsing#v:position).

See also:

• Python-like syntax #1139