Refine the error type in parser.ts

[baetheus]

The initial Parser type in parser.ts reuses the DecodeError type from decoder.ts. It does this because there was already a bevy of combinators for creating and merging various DecodeError structures (intersection, union, leaf, etc) and I was unclear on the structure of errors I might encounter in designing parser.ts.

It has become clear that this implementation of parser really only has a one or more errors and that the errors should include cursor and length information to create better error messages.

Additionally, I want to look into partial parse results so recovery is an option.