Overhaul Span to track start/end positions instead of offset+length

[saiashirwad]

The current Span type is stored as a single point plus a byte length:

type Span = {
  offset: number
  length: number
  line: number
  column: number
}

This is fine for simple single-line errors, but it starts to break down once we want:

• Multi-line spans (e.g. an unclosed string that runs for several lines)
• Related spans ("unclosed ( opened here")
• Clean conversion to LSP / JSON / SARIF output formats
• Span unions and precise end positions

I'd like to move to a start/end position model:

type Position = {
  byteOffset: number
  charOffset: number
  line: number
  column: number
}

type Span = {
  start: Position
  end: Position
}

byteOffset stays for fast source.slice(...), and charOffset/column become user-facing. This is a breaking change to the public Span shape, so it should be done before a 1.0 release if possible.

Open questions:

• Should we keep the old Span(state, length) factory for backwards compatibility?
• Should Span carry a cached source slice, or keep slicing on demand in the formatter?
• How does this affect Parser.spanned() and the internal Span() helper in errors.ts?