Capture diagnostics to zip file (#1656)

* Capture diagnostics to zip file

The `captureDiagnostics` command would capture many log files and place
them in a temporary folder, then present that folder to the user. The
most common use case for capturing diagnostics is to share them, so zip
the diagnostics up in to a more easily sharable .zip file.

Adds some tests to flesh out coverage on `captureDiagnostics`.

* Update CHANGELOG

Author: plemarquand

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 - Cleanup Swift diagnostics when the source file is moved or deleted ([#1653](https://github.com/swiftlang/vscode-swift/pull/1653))
 - Make sure newline starts with /// when splitting doc comment ([#1651](https://github.com/swiftlang/vscode-swift/pull/1651))
+- Capture diagnostics with `Swift: Capture Diagnostic Bundle` to a .zip file ([#1656](https://github.com/swiftlang/vscode-swift/pull/1656))
 - Prevent continuous "package resolve" cycles ([#1654](https://github.com/swiftlang/vscode-swift/pull/1654))
 - Fix error when running `Reset Package Dependencies` command from the Project view ([#1661](https://github.com/swiftlang/vscode-swift/pull/1661))