The `DataTable` widget now takes the `by` argument instead of `columns`, allowing the table to also be sorted using a custom function (or other callable). This is a breaking change since it requires all calls to the `sort` method to include an iterable of key(s) (or a singular function/callable). Covers Textualize#2261 using [suggested function signature](Textualize#2512 (comment)) from @darrenburns on PR Textualize#2512.

Changed back to orinal `columns` argument and added a new `key` argument
which takes a function (or other callable). This allows the PR to NOT BE
a breaking change.

- Updated the example file for the docs to better show the functionality
of the change (especially when using `columns` and `key` together).
- Added one new tests to cover a similar situation to the example
  changes

- the sort by clicked column function was bloat in my opinion

…s test coverage of the case where a key, but no columns, is passed to DataTable.sort.

… PR link