Add method to convert ClassifiedBlock into completion locations. (#2316)

The completion engine maps completion locations to spans on a line, which
indicate whther to complete a command name, flag name, argument, and so on.

Initial implementation is simplistic, with some rough edges, since it relies
heavily on the parser's interpretation. For example

    du -

if asking for completions, `-` is considered a positional argument by the
parser, but the user is likely looking for a flag. These scenarios will be
addressed in a series of progressive enhancements to the engine.

crates/nu-cli/src/cli.rs

@@ -7,7 +7,6 @@ use crate::context::Context;
 use crate::git::current_branch;
 use crate::path::canonicalize;
 use crate::prelude::*;
-use crate::shell::completer::NuCompleter;
 use crate::shell::Helper;
 use crate::EnvironmentSyncer;
 use futures_codec::FramedRead;
@@ -787,10 +786,7 @@ pub async fn cli(
 
         let cwd = context.shell_manager.path();
 
-        rl.set_helper(Some(crate::shell::Helper::new(
-            Box::new(<NuCompleter as Default>::default()),
-            context.clone(),
-        )));
+        rl.set_helper(Some(crate::shell::Helper::new(context.clone())));
 
         let colored_prompt = {
             if let Some(prompt) = config.get("prompt") {