This commit adds the InBackground and ShortCircuitOnFailure attributes
to commands, which respectively cause the command to be run in the
background, and abort the command chain with exit status != 0.
Prior to this, we did not care if there was any whitespace after the
last token in the prompt, and this caused a regression:
```
> lsp <tab>
> lsp ci
```
To achieve this, the API was tweaked a bit to allow for easier tracking
of completions.
This API change is non-disruptive to any application that does not use
anchored styles.
LibLine should ultimately not care about what a "token" means in the
context of its user, so force the user to split the buffer itself.
This also allows the users to pick up contextual clues as well, since
they have to lex the line themselves.
This commit pacthes Shell and the JS repl to better handle completions,
so certain wrong behaviours are now corrected as well:
- JS repl can now complete "Object . getOw<tab>"
- Shell can now complete "echo | ca<tab>" and paths inside strings
