* make sort-by fail gracefully if mismatched types are compared
* Added a test to check if sorted-by with invalid types exists gracefully
* Linter changes
* removed redundant pattern matching
* Changed the error message
* Added a comma after every argument
* Changed the test to accomodate the new err messages
* Err message for sort-by invalid types now shows the mismatched types
* Lints problems
* Changed unwrap to expect
* 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).
