Become more robust to parse errors

[rockstar]

I've started working on a sql lsp server implementation, using sqlparser as the parser. Unfortunately, I very quickly hit a wall, mostly around how sqlparser discovers and surfaces errors. For instance, let's say the user types the following: SELECT * F and hits the key. A good lsp server would suggest completing F to FROM, even just do it automatically in that case. If one tries to parse that input string with sqlparser, it returns a ParserError("Expected end of statement, found: F"). The error isn't unexpected, but now (a) since the ast isn't available in the face of errors, there are no recommendations/commands/lints I can make based on a error-prone AST, and (b) I don't have enough information (line/col info) to send a diagnostic (the thing that makes the red squiggly lines) to the client. The line/col information was suggested in #179, but a fair amount of the work that would enable a really good LSP server implementation is being able to work with an AST that has errors in it.

I suspect what I'm describing is really hard, and has all sorts of backwards-incompatible changes that would be required to make this work. I think it'd be worthwhile, though.

[alamb]

Possibly related to #524