remove .vscode and gitignore it (#4498)

**What type of PR is this?**

Other

**What does this PR do? Why is it needed?**

The .vscode directory isn't meant to be checked in like this, and causes
churn that vscode users have to remove before sending PRs. We remove it,
add it to .gitignore so future folks don't accidentally check it in.
Folks can check out the more general Editor setup wiki page for
instructions.

**Which issues(s) does this PR fix?**

Fixes #4497.

Author: jmhodges

.gitignore

@@ -5,3 +5,4 @@
 /examples/*/bazel-*
 /.ijwb/
 /tests/bcr/.ijwb/
+.vscode

.vscode/extensions.json

@@ -2,6 +2,7 @@ Want to contribute? Great! First, read this page (including the small print at
 the end).
 
 ### Before you contribute
+
 **Before we can use your code, you must sign the
 [Google Individual Contributor License Agreement](https://developers.google.com/open-source/cla/individual?csw=1)
 (CLA)**, which you can do online.
@@ -15,6 +16,7 @@ the CLA until after you've submitted your code for review and a member has
 approved it, but you must do it before we can put your code into our codebase.
 
 ### The small print
+
 Contributions made by corporations are covered by a different agreement than
 the one above, the
 [Software Grant and Corporate Contributor License Agreement](https://cla.developers.google.com/about/google-corporate).