* Show error when trying to sort by invalid column
* Added test for changes
* Addressed comments, updated test
* Removed unnecessary mutable keyword
* Changed split-column to solt column after rebase from upstream
This improves incremental build time when working on what was previously
the root package. For example, previously all plugins would be rebuilt
with a change to `src/commands/classified/external.rs`, but now only
`nu-cli` will have to be rebuilt (and anything that depends on it).
