Make ParsingError non-exhaustive in v0.33.0

`miden_assembly_syntax::parser::ParsingError` is a public, exhaustive enum. Mark it `#[non_exhaustive]` in v0.33.0. Downstream matches will then require a fallback arm, so later variants can be added without breaking client builds.

[PR #3812](https://github.com/0xMiden/miden-vm/pull/3812) records why this is needed. It restored the v0.32.0 enum layout after new variants broke the v0.32.1 patch release.