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-source/src/meta.rs

@@ -526,10 +526,11 @@ impl Span {
     /// let span = Span::new(2, 8);
     ///
     /// assert_eq!(span.contains(5), true);
+    /// assert_eq!(span.contains(8), false);
     /// assert_eq!(span.contains(100), false);
     /// ```
     pub fn contains(&self, pos: usize) -> bool {
-        self.start <= pos && self.end >= pos
+        self.start <= pos && pos < self.end
     }
 
     /// Returns a new Span by merging an earlier Span with the current Span.