Make sure completions work even after an operator / parentheses

For example for a query that starts a function parentheses:

```sql
FROM index | WHERE TRIM(
^
```

we should be getting completions that preserve the closing parenthesis.

For queries like:

```sql
FROM index
| EVAL g = 1, a=1+
^
```

we should get and complete g even if we haven't typed a space after the +

## How

This might involve building either a collection of all symbols the parser can have (look at https://github.com/neo4j/cypher-language-support/blob/d5191e5a140d0f8153d5bd343db96c440e083b63/packages/language-support/src/lexerSymbols.ts for inspiration) and maybe considering whether the caret for the completion was moved back or not.