Add related spans and suggested fixes to ParseError

[saiashirwad]

Currently a ParseError is a single point with a message. It works for "expected ; here", but it can't express richer diagnostics like:

• "Unclosed (" with a related span showing where it was opened
• "Missing closing quote" with a suggested insertion at the cursor
• "Did you mean let?" as an actual editor fix, not just a hint message

I'd like to extend the error types with two lightweight fields:

type RelatedSpan = {
  message: string
  span: Span
}

type SuggestedFix = {
  description: string
  edits: { span: Span; replacement: string }[]
}

Then ParseError variants can carry related?: RelatedSpan[] and fixes?: SuggestedFix[].

This keeps us out of full compiler-diagnostic territory (no severity levels, no warnings) while still enabling much better error messages and future IDE / LSP integration. It pairs naturally with the Span overhaul in #X and the formatter refactor in #Y.

One thing to watch: when we merge errors from or(), we should probably drop related spans/fixes from non-primary attempts so the final message isn't noisy.