api to support call hierarchy view

[jrieken]

This is for #16110, ideas already exist in microsoft/vscode-languageserver-node#420

[jrieken]

After discussions on microsoft/vscode-languageserver-node#420, I am distilling something like this:

enum CallHierarchyDirection {
    CallsFrom,
    CallsTo,
}

class CallHierarchyItem {
    kind: vscode.SymbolKind;
    name: string;
    detail?: string;
    uri: vscode.Uri;
    range: vscode.Range;
    selectionRange: vscode.Range;
}

interface CallHierarchyItemProvider {

    provideCallHierarchyItem(
        document: vscode.TextDocument,
        postion: vscode.Position,
        token: vscode.CancellationToken
    ): vscode.ProviderResult<CallHierarchyItem>;

    resolveCallHierarchyItem(
        item: CallHierarchyItem,
        direction: CallHierarchyDirection,
        token: vscode.CancellationToken
    ): vscode.ProviderResult<[CallHierarchyItem, vscode.Location[]][]>;
}

[aeschli]

interface CallHierarchyItemProvider {
    /** 
     * Evaluates the symbols(s) defined (or referenced) at given position, and 
     * returns all incoming or outgoing calls to the symbol(s).
     */
    provideCalls(doc: TextDocument, pos: Position | Range, dir: CallHierarchyDirection) : Promise<CallHierarchyCall[]>;

}

enum CallHierarchyDirection {
    CallsFrom,
    CallsTo,
}

class CallHierarchyCall {
   /** The source range of the reference. The range is a sub range of the 'from' symbol range */
   range: Range;

   /** The symbol that contains the reference */
   from: CallHierarchySymbol;

   /** The symbol that is referenced */
   to: CallHierarchySymbol; 
}

class CallHierarchySymbol {
    kind: SymbolKind;
    name: string;
    detail?: string;
    uri: Uri;
    range: Range;
    selectionRange: Range;
}