[vscode] Add .vscode directory to .gitignore EXCEPT extensions.json

Summary:
We generally want to ignore the .vscode directory. However, we decided extensions.json is something we want to include in the repository... and so we should exclude it from being ignored.

(Working off of the example here: https://stackoverflow.com/a/16318111)

Test Plan:

1. Add a random file to the .vscode directory
2. Make sure the file doesn't show up in the staging area on git status
3. Make a random change to .vscode/extensions.json
4. Make sure that the random change does show up in staging area on git status

Reviewers: benschac, ashoat

Reviewed By: benschac, ashoat

Subscribers: palys-swm, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3442