Add more links to the README

[AlexWaygood]

Summary

I decided recently it was finally time to dogfood ty to myself in VSCode. Two things confused me while installing the extension:

• I searched for "ty-vscode" in the marketplace, and no results came up. The reason is that the extension is actually called "ty", not "ty-vscode". I've added a link to the VSCode marketplace so that this is obvious to anybody reading the README.
• The README recommends to edit your settings.json file, but I'm a VSCode noob and had no idea how to do that. I've added a link to the settings.json reference so that other people in a similar boat don't have to look it up manually

[Gankra]

Thanks!