Allow link provider results to get merged if the link range is identical

[Tyriar]

In order to facilitate something like being able to use multiple link providers via a tooltip rather than just the default, we should expose links that match the range of another link exactly through the API.

VS Code issue: microsoft/vscode#97070

[Tyriar]

Possible proposal:

interface ILink {
  // Filled in by xterm.js
  mergedLinks?: ILink[];
}

This is a little weird as it's a property not intended to be provided by the embedder. Another alternative:

interface ILinkProvider {
  provideLinks(bufferLineNumber: number, callback: (links: ILink[] | undefined) => IResolvedLink[] | undefined): void;
}
interface IResolvedLink extends ILink {
  mergedLinks?: ILink[];
}

This would allow the embedder to grab the resolved links from the callback function that would fill things in.