Proposed expandable hover API

[gabritto]

VSCode counterpart: microsoft/vscode#228255

This is an incomplete prototype of expandable hovers using the [current vscode proposed expandable hover API].

Mainly the only scenario implemented so far is for hovering over variables, e.g.:

type A = number | string;
type B = symbol | boolean;
type SomeType<T, U> = {
    propA: T,
    propB: U,
};

let a: A; // Hover on `a` will be expandable
let b: B; // Hover on `b` will be expandable
let someVar: SomeType<A, B>; // Hover on `someVar` will be expandable

Also not implemented yet is computing the upper limit of verbosity level. This means you'll always be able to click the + button on the UI, even if nothing will be expanded.

[typescript-bot]

Thanks for the PR! It looks like you've changed the TSServer protocol in some way. Please ensure that any changes here don't break consumers of the current TSServer API. For some extra review, we'll ping @sheetalkamat, @mjbvz, @zkat, and @joj for you. Feel free to loop in other consumers/maintainers if necessary.

[typescript-bot]

Looks like you're introducing a change to the public API surface area. If this includes breaking changes, please document them on our wiki's API Breaking Changes page.

Also, please make sure @DanielRosenwasser and @RyanCavanaugh are aware of the changes, just as a heads up.