Prevent changing cwd to a file

Fixes #110.

package.json

@@ -1,6 +1,6 @@
 {
     "name": "fwdekker.com",
-    "version": "0.28.0",
+    "version": "0.28.1",
     "description": "The source code of [my personal website](https://fwdekker.com/).",
     "author": "Felix W. Dekker",
     "repository": {

src/main/js/Commands.ts

@@ -365,7 +365,7 @@ export class Commands {
         }
 
         const path = Path.interpret(this.environment.get("cwd"), input.args[0]);
-        if (!this.fileSystem.has(path)) {
+        if (!this.fileSystem.has(path) || !(this.fileSystem.get(path) instanceof Directory)) {
             streams.err.writeLine(`cd: The directory '${path}' does not exist.`);
             return -1;
         }