[plugin] contribute.keybindings can be objects or arrays.

[svenefftinge]

What it does

Aligns with VS Code which allows to use both single raw keybindings or arrays of them in package.json contributions.

How to test

Install an extension that doesn't wrap a single keybinding in an array, like e.g. https://github.com/Cmacu/gotoanything/blob/master/package.json#L65

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[svenefftinge]

Because it is the second time we had such an incompatibility, I also made the code more robust by wrapping the individual contributions in try-catch. So that a single issue doesn't leave the extension half initialized and we get a better error message.

[akosyakov]

code changes look good, tested that a keybinding as an object is registered