(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/ui2018/search.ts

@@ -128,8 +128,8 @@ export function searchBar(model: SearchModel, testId: TestId = noTestId) {
     find: () => { inputElem.focus(); inputElem.select(); },
     // On Mac, Firefox has a default behaviour witch causes to close the search bar on Cmd+g and
     // Cmd+shirt+G. Returning false is a Mousetrap convenience which prevents that.
-    findNext: () => {model.findNext().catch(reportError); return false; },
-    findPrev: () => {model.findPrev().catch(reportError); return false; },
+    findNext: () => { model.findNext().catch(reportError); return false; },
+    findPrev: () => { model.findPrev().catch(reportError); return false; },
   }, null, true);
 
   const toggleMenu = debounce((_value?: boolean) => {