(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/ui/PageWidgetPicker.ts

@@ -196,7 +196,7 @@ export function buildPageWidgetPicker(
     dom.create(PageWidgetSelect, value, tables, columns, onSaveCB, options),
 
     // gives focus and binds keydown events
-    (elem: any) => {setTimeout(() => elem.focus(), 0); },
+    (elem: any) => { setTimeout(() => elem.focus(), 0); },
     onKeyDown({
       Escape: () => ctl.close(),
       Enter: () => isValid() && onSaveCB()