Shell: Add 'match' expressions

This commit adds an equivalent to the sh 'case' construct, except it's
much more pleasing to look at and write:
```sh
match "$something" {
    p1 { echo "p1!" }
    p2 { echo "p2!" }
    *  { echo "string catch-all!" }
}
```
is the equivalent of:
```sh
case $something in
    p1)
        echo "p1!"
        ;;
    p2)
        echo "p2!"
        ;;
    *)
        echo "catch-all!"
        ;;
esac
```

Since our shell does not treat lists as strings, matching lists is also
possible:

```sh
match (1foo 2foo foo3) {
    (?foo 2* *) { echo wowzers! }
    (* * *) { echo 3-element list catch-all }
}
```

Shell/AST.h

@@ -757,6 +757,31 @@ private:
     RefPtr<Node> m_right;
 };
 
+struct MatchEntry {
+    NonnullRefPtrVector<Node> options;
+    Vector<Position> pipe_positions;
+    RefPtr<Node> body;
+};
+
+class MatchExpr final : public Node {
+public:
+    MatchExpr(Position, RefPtr<Node> expr, String name, Optional<Position> as_position, Vector<MatchEntry> entries);
+    virtual ~MatchExpr();
+
+private:
+    virtual void dump(int level) const override;
+    virtual RefPtr<Value> run(RefPtr<Shell>) override;
+    virtual void highlight_in_editor(Line::Editor&, Shell&, HighlightMetadata = {}) override;
+    virtual HitTestResult hit_test_position(size_t) override;
+    virtual String class_name() const override { return "MatchExpr"; }
+    virtual bool would_execute() const override { return true; }
+
+    RefPtr<Node> m_matched_expr;
+    String m_expr_name;
+    Optional<Position> m_as_position;
+    Vector<MatchEntry> m_entries;
+};
+
 class Or final : public Node {
 public:
     Or(Position, RefPtr<Node>, RefPtr<Node>);