Align `Assignment`/`Function` distinction in AST with compiler's implemented semantics:
- The ambiguous case `funcOrVar = expression` is now parsed as a `Function` when in a `Type` definition or in the top level of a module. I.e. it is an `Assignment` in contexts where the RHS is evaluated immediately when the binding is evaluated, and a `Function` in contexts where the RHS is evaluated each time the bound name is evaluated.
- `Assignment` statements now may only occur in function bodies.
Correcting this distinction lays the groundwork for #11302.
Other changes:
- Fixed incorrect source code locations for negative literals and negated expressions.
# Important Notes
New APIs:
- The parser now exposes a `parse_block` entry point, which allows parsing input lines as if in the body of a function. The previous entry point has been renamed to `parse_module`.
# Important Notes
The command to run the gui dev environment has been changed. Invoking the old command will print a message about that.
From now on, use `pnpm dev:gui2` in repository root.
