Elminate implicit cd

[krader1961]

Replace the CLI implicit cd behavior with location mode support for literal directory paths.

Fixes #1099

[krader1961]

Force push to fix the conflict with the change from the 0.20.0-release-notes.md to the 0.21.0-release-notes.md file since this PR was initially created. No other changes were made.

[xiaq]

After playing with this a bit, it feels non-ideal:

• You can't trigger a tab completion from location mode - I realize that I've already mentioned this in Keep and document implicit cd? The / (division command) invoked with no args does cd / #1099 (comment), but now that I've played with it seems that I almost always use tab completion with implicit cd.

• ~ doesn't work in location mode (this should be fixable though)

I now feel literal cd via location mode may not be the best idea. Maybe adding a completer for cd to only show directories and fixing #474 would be a better UX, albeit with slightly more keypresses.

As for the removal of implicit cd, I'd rather deprecate it first and only remove it after the next release.

The headings in tour.md were chosen intentionally to reflect the general functionality rather than Elvish-specific terms, please don't change them.

Thanks for implementing this, and sorry that I've changed my mind. I'll close this PR and add deprecation for implicit cd.

[krader1961]

I now feel literal cd via location mode may not be the best idea. Maybe adding a completer for cd to only show directories and fixing #474 would be a better UX, albeit with slightly more keypresses.

That is actually what I would prefer, @xiaq. I only did this as an experiment and to gain more experience writing interactive unit tests. I've always maintained that implict cd isn't worth saving just two keystrokes compared to this pull-request. However, if those two keystrokes are worth eliminating it might be worth exploring how to make tab completion work in location mode. Also, the fact my change doesn't handle a ~/ prefix was an oversight and could probably be handled easily.