[chore] add recommended extensions for vscode users

Summary: A recommended extensions for vscode users. Initial addition adds Editor config (https://marketplace.visualstudio.com/items?itemName=EditorConfig.EditorConfig) package to recommended list. Followed steps here: https://dev.to/askrishnapravin/recommend-vs-code-extensions-to-your-future-teammates-4gkb

Test Plan: Download project, open vscode, recommend prompt should appear in the bottom corner.

Reviewers: atul, ashoat

Reviewed By: atul

Subscribers: palys-swm, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3428