Carry exact expected statement type in `ParserError`

Currently `sqlparser-rs` return a `&str` to tell which type of token parser is expecting:
``` rust
fn expected(&self, expected: &str, found: Token) -> Result
```

When a SQL string is not completed.
Return an exact `ExpectType` will be better, this can deliver help for developing tools like SQL complete tools etc.
``` rust
fn expected(&self, expected: Vec, found: Token) -> Result
```

Start by locating the ParserError implementation and the expected function described in the issue, then inspect how incomplete SQL is parsed. Define what ExpectType values must represent and verify that exact expectations are preserved for SQL completion tooling, with coverage for incomplete statements.

Written by the indexing model from the issue text.