(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

test/nbrowser/gristUtils.ts

@@ -1333,8 +1333,9 @@ export class Session {
     const api = this.createHomeApi();
     if (!noCleanup) {
       cleanup.addAfterEach(async () => {
-        if (doc.id)
+        if (doc.id) {
           await api.deleteDoc(doc.id).catch(noop);
+        }
         doc.id = '';
       });
     }