Build parser for Rib import statements

[afsalthaj]

This is the first step to first class worker support in Rib

Build a parser for import statements with tests, and the first one that we would be supporting is just functions, and therefore the grammar can be quite biased to just importing functions. We will deal with importing any other types later

Grammar

import_statement = "import", function_name, ["as", identifier];

Grammar may differ as we go, to bring in component name (package) in it or not

We will reuse the parser for function_name.

It can get materialized into a module. For now we just assume its just a single module, but good to go on this route straight away. Import will not be considered as an Expr, as it can lead to hefty complexities such as InferredType

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Module {
  pub imports: Vec<Import>,
  pub expressions: Vec<Expr>, // Or statements, depending on design.
}