Added support for Visual Studio 2019

[madskristensen]

Here's what I did:

• Updated InstallationTarget version range to include VS 2019 (16.0)
• Removed redundant InstallationTargets (Pro and Enterprise) since Community covers them both
• Made dependency on CoreEditor open ended with a minimum version of 15.0

If you have any questions, please let me know.

When/if you take these changes and uploads the new version to the Marketplace, don't be alarmed that the Marketplace doesn't yet show that the extension supports VS 2019. Please upload regardless.

Mads Kristensen Visual Studio Extensibility Team

[madskristensen]

Looks like there are issues with the test project. I didn't touch anything that could have affected it, so I assume the issue was there already when I sent this PR