(core) Add rules to eslint to better match our coding conventions.

Summary:
We used tslint earlier, and on switching to eslint, some rules were not
transfered. This moves more rules over, for consistent conventions or helpful
warnings.

- Name private members with a leading underscore.
- Prefer interface over a type alias.
- Use consistent spacing around ':' in type annotations.
- Use consistent spacing around braces of code blocks.
- Use semicolons consistently at the ends of statements.
- Use braces around even one-liner blocks, like conditionals and loops.
- Warn about shadowed variables.

Test Plan: Fixed all new warnings. Should be no behavior changes in code.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2831

app/client/components/Comm.ts

@@ -204,15 +204,16 @@ export class Comm extends dispose.Disposable implements GristServerAPI, DocListA
   public pendingRequests: Map<number, CommRequestInFlight>;
   public nextRequestNumber: number = 0;
 
+  protected listenTo: BackboneEvents["listenTo"];            // set by Backbone
+  protected trigger: BackboneEvents["trigger"];              // set by Backbone
+  protected stopListening: BackboneEvents["stopListening"];  // set by Backbone
+
   // This is a map from docId to the connection for the server that manages
   // that docId.  In classic Grist, which doesn't have fixed docIds or multiple
   // servers, the key is always "null".
   private _connections: Map<string|null, GristWSConnection> = new Map();
   private _collectedUserActions: UserAction[] | null;
   private _singleWorkerMode: boolean = getInitialDocAssignment() === null;  // is this classic Grist?
-  private listenTo: BackboneEvents["listenTo"];            // set by Backbone
-  private trigger: BackboneEvents["trigger"];              // set by Backbone
-  private stopListening: BackboneEvents["stopListening"];  // set by Backbone
 
   public create() {
     this.autoDisposeCallback(() => {