[Feature] Add RustRover .idea directory to .gitignore file

[hdwalters]

I use RustRover for Rust development. This creates a hidden directory .idea, which shows up as an untracked file in the git status output:

$ git status
On branch master
Your branch is up-to-date with 'origin/master'.

Untracked files:
  (use "git add <file>..." to include in what will be committed)
	.idea/

nothing added to commit but untracked files present (use "git add" to track)

I am concerned that this directory might get committed by accident, so it may be sensible to add it to the .gitignore file. When I did this in #430, b1ek suggested that:

this is a topic for another discussion, please remove it from this PR
it might contain important data that is better to be shared between developers (like .cargo)

I have backed out that commit, but does anyone have strong views on this?

[b1ek]

i think that the deciding factor in this would be if there's a lot of RustRover users who are also active contributors.

i just don't see a lot of sense in creating a config for a thing if no one will benefit from it

(and of course if we don't need that it should be gitignored)

[b1ek]

see also: What to gitignore from the .idea folder?