Add common editor config directories to .gitignore (dfinity#3264)

Ignores project config files and directories generated by VSCode, IntelliJ-based IDEs, Eclipse, Vim, etc. 

Adding as a PR since this tends to be a reasonably common practice for open-source repositories. These specific entries are from the `.gitignore` in [rust-lang/rust](https://github.com/rust-lang/rust/blob/master/.gitignore#L13-L23).

.gitignore

@@ -12,3 +12,15 @@ result*
 /samples/**/*.txt
 
 enable-internals
+
+# Editor configuration
+*.swp
+*.swo
+Session.vim
+*.iml
+.idea
+.vscode
+.project
+.cproject
+.favorites.json
+.settings/