(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/models/SearchModel.ts

@@ -92,7 +92,7 @@ export class SearchModelImpl extends Disposable implements SearchModel {
     this.autoDispose(this.value.addListener(v => { void findFirst(v); }));
 
     // Set this.noMatch to false when multiPage gets turned ON.
-    this.autoDispose(this.multiPage.addListener(v => { if (v) { this.noMatch.set(false); }}));
+    this.autoDispose(this.multiPage.addListener(v => { if (v) { this.noMatch.set(false); } }));
 
     // Schedule a search restart when user changes pages (otherwise search would resume from the
     // previous page that is not shown anymore). Also revert noMatch flag when in single page mode.