Should the `generic` dialect support numeric literals with underscore separators?

Right now, the `generic` dialect (DataFusion's default) does not support numeric literals with separators:

```sql
DataFusion CLI v54.0.0
> select 1_000;
+------+
| _000 |
+------+
| 1 |
+------+
```

I'm wondering if the `generic` dialect should support this, just like Postgres/DuckDB/SQLite/Clickhouse, so DataFusion can use them directly without needing to switch to another dialect.

Start by reproducing `select 1_000;` in the DataFusion CLI and trace how the `generic` dialect tokenizes and parses numeric literals. Compare its behavior with the Postgres, DuckDB, SQLite, or ClickHouse dialects mentioned in the issue; done means the generic dialect accepts numeric separators without treating `_000` as a column name.

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