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/Parser.h

@@ -185,7 +185,9 @@ subshell :: '{' toplevel '}'
 
 match_expr :: 'match' ws+ expression ws* ('as' ws+ identifier)? '{' match_entry* '}'
 
-match_entry :: match_pattern ws* '{' toplevel '}'
+match_entry :: match_pattern ws* (as identifier_list)? '{' toplevel '}'
+
+identifier_list :: '(' (identifier ws*)* ')'
 
 match_pattern :: expression (ws* '|' ws* expression)*