Meta: Ignore auto-generated and incompatible files in prettier

Several auto-generated Markdown files should be
excluded once we check Markdown. One JS regex test
isn't compatible with prettier's parser.

.prettierignore

@@ -1,7 +1,13 @@
+# Automatically regenerated Markdown files based on git patches.
+Toolchain/Patches
+Ports/*/patches
+
 Base/home/anon/Source/js
 Userland/Libraries/LibJS/Tests/invalid-lhs-in-assignment.js
 Userland/Libraries/LibJS/Tests/unicode-identifier-escape.js
 Userland/Libraries/LibJS/Tests/modules/failing.mjs
+# FIXME: Uses regex flags which cause a syntax error in prettier.
+Userland/Libraries/LibJS/Tests/builtins/RegExp/RegExp.prototype.flags.js
 
 # FIXME: Remove once prettier is updated to support using declarations.
 Userland/Libraries/LibJS/Tests/builtins/DisposableStack/DisposableStack.prototype.@@dispose.js

Userland/Libraries/LibJS/Tests/builtins/RegExp/RegExp.prototype.flags.js

@@ -5,6 +5,7 @@ test("basic functionality", () => {
     expect(/foo/i.flags).toBe("i");
     expect(/foo/m.flags).toBe("m");
     expect(/foo/s.flags).toBe("s");
+    // prettier-ignore
     expect(/foo/v.flags).toBe("v");
     expect(/foo/u.flags).toBe("u");
     expect(/foo/y.flags).toBe("y");