Shell: Allow parts of globs to be named in match expressions

This patchset allows a match expression to have a list of names for its
glob parts, which are assigned to the matched values in the body of the
match.
For example,
```sh
stuff=foobarblahblah/target_{1..30}
for $stuff {
    match $it {
        */* as (dir sub) {
            echo "doing things with $sub in $dir"
            make -C $dir $sub # or whatever...
        }
    }
}
```

With this, match expressions are now significantly more powerful!

Shell/AST.h

@@ -912,6 +912,8 @@ private:
 
 struct MatchEntry {
     NonnullRefPtrVector<Node> options;
+    Optional<Vector<String>> match_names;
+    Optional<Position> match_as_position;
     Vector<Position> pipe_positions;
     RefPtr<Node> body;
 };