Parse out error location in file to only highlight the word reported by FTL

[wesbillman]

There's a basic implementation in main now, but we should update it to be more efficient and robust. The basic version implemented in lsp should only be used if we don't get EndColumn information from the solutions below.

There are likely 2 options here.

1. Add an EndColumn field to schema.Position and update as needed
2. Add EndColumn to custom Error struct

[wesbillman]

We decided on using a custom error type for this and generating protos for that error type so that other runtimes can have a consistent interface for these errors.