Enables formatting `Expr::Column` by adding the context to `ToTokens`
instead of creating a new unparsing implementation for each node.
`ToTokens` implemented for:
- [x] `UpdatePlan`
- [x] `Plan`
- [x] `JoinedTable`
- [x] `SelectPlan`
- [x] `DeletePlan`
Reviewed-by: Pedro Muniz (@pedrocarlo)
Closes#1949