* Refactor all completion logic into `NuCompleter`
This is the next step to improving completions. Previously, completion logic was
scattered about (`FilesystemShell`, `NuCompleter`, `Helper`, and `ShellManager`).
By unifying the core logic into a central location, it will be easier to take the
next steps in improving completion.
* Update context.rs
Co-authored-by: Jonathan Turner <jonathandturner@users.noreply.github.com>
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