doddish scanner: modernize error handling and operator model

[friedenberg]

Collected TODOs from go/internal/_/doddish/scanner.go that represent design improvements to the scanner.

Items

1. Switch err field to pkgError (line 21) — The scanner's err field is error but should use the package's typed error system. Not a simple field type change since the scanner stores io.EOF and errors.Wrap() results that don't implement GetErrorType(). Needs error wrapping at the boundary (e.g., in Error() method) or a scanner-specific error wrapper.

2. Remove UnreadRune entirely (line 65) — Replace UnreadRune with a different buffering strategy.

3. Create sequence methods for Scan family (line 110) — Add sequence-returning variants of the Scan methods.

4. Make quotes and backslash operators (line 174) — Promote quote and backslash characters to first-class Op types rather than special-casing them in the scan loop.

5. Add support for ellipsis (lines 263, 308) — Support ... syntax in consumeLiteralOrFieldValue and consumeIdentifierLike.