(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/declarations.d.ts

@@ -45,9 +45,9 @@ declare module "app/client/components/BaseView" {
   import {DomArg} from 'grainjs';
   import {IOpenController} from 'popweasel';
 
-  type Options = {
-    init? : string,
-    state? : any
+  interface Options {
+    init?: string;
+    state?: any;
   }
 
   namespace BaseView {}
@@ -68,7 +68,7 @@ declare module "app/client/components/BaseView" {
     public createFilterMenu(ctl: IOpenController, field: ViewFieldRec, onClose?: () => void): HTMLElement;
     public buildTitleControls(): DomArg;
     public getLoadingDonePromise(): Promise<void>;
-    public activateEditorAtCursor(options?: Options) : void;
+    public activateEditorAtCursor(options?: Options): void;
     public onResize(): void;
     public prepareToPrint(onOff: boolean): void;
     public moveEditRowToCursor(): DataRowModel;