(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/GristWSConnection.ts

@@ -106,6 +106,8 @@ export class GristWSConnection extends Disposable {
   public useCount: number = 0;
   public on: BackboneEvents['on'];    // set by Backbone
 
+  protected trigger: BackboneEvents['trigger']; // set by Backbone
+
   private _clientId: string|null;
   private _clientCounter: string;     // Identifier of this GristWSConnection object in this browser tab session
   private _assignmentId: string|null;
@@ -118,7 +120,6 @@ export class GristWSConnection extends Disposable {
   private _reconnectAttempts: number = 0;
   private _wantReconnect: boolean = true;
   private _ws: WebSocket|null = null;
-  private trigger: BackboneEvents['trigger']; // set by Backbone
 
   constructor(private _settings: GristWSSettings = new GristWSSettingsBrowser()) {
     super();