add 'stagehand' command and navigation keys

- adds 'stagehand' command to command palette
- adds shell-like navigation keys for future fs navigation ui
- adds 'kill to eol' command
add CirtularArray data structure
add syntax def for fs ui navigation buffer

A specific syntax def will enable us to limit some key bindings
to the fs navigation widget.
add support code and tests for fs navigation
fix #185 - integrate with stagehand

Stagehand integration works like a somewhat primitive wizard:

1) select 'Dart: Stagehand' command
2) select template name
3) select target directory

If stagehand is not installed, it will be installed and the
template generation will be aborted. The user will have to
restart the process in order to generate a template.
add tests; refactor
improve stagehand integration
open project in ST after it's been created
change property name
add ability to cycle completions backward
add tests
Merge pull request #404 from guillermooo/new-185-stagehand-integration

fix #185 stagehand integration