AST: Return analysis errors through DiagnosticManager

[LesterEvSe]

Description

This PR prepares the AST phase for reporting multiple errors. Program::analyze now takes &mut DiagnosticManager and returns Option<Self> instead of Result<Self, Diagnostic>. Behaviour does not change: analysis still stops at the first error.

Next steps

Follow-up PRs will extend analysis from one error to many, one level at a time:

1. Report every error inside an expression (tuple elements, call arguments, match arms), not only the first one. Report every error inside an expression #420
2. Continue with the next item after an item fails. Keep analyzing items after a function body fails #424
3. Continue with the next statement after a statement fails.

Each step reports only independent errors, so it does not need poisoning to prevent cascading errors. Poisoning will come later, for the cases where analysis would otherwise stop, such as a broken declared type.

[KyrylR]

ACK 1f279a4; successfully ran local tests