[cleanup] Add .idea to .gitignore

When you use the IntelliJ IDE, all the project-specific
settings for the project are stored under the .idea folder.
This patch add .idea to .gitignore to support IntelliJ IDE.

Change-Id: I55ca6530f00fd64f088485f7c856ad602b145f63
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3813066
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82637}

.gitignore

@@ -30,6 +30,7 @@
 .cproject
 .gclient_entries
 .gdb_history
+.idea
 .jslint-cache
 .landmines
 .project