(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/DocMenu.ts

@@ -245,8 +245,8 @@ function buildWorkspaceDocBlock(home: HomeModel, workspace: Workspace, flashDocI
           // The flash value may change to true, and then immediately to false. We highlight it
           // using a transition, and scroll into view, when it turns back to false.
           transition(flash, {
-            prepare(elem, val) { if (!val) { elem.style.backgroundColor = colors.slate.toString(); }},
-            run(elem, val) { if (!val) { elem.style.backgroundColor = ''; scrollIntoViewIfNeeded(elem); }},
+            prepare(elem, val) { if (!val) { elem.style.backgroundColor = colors.slate.toString(); } },
+            run(elem, val) { if (!val) { elem.style.backgroundColor = ''; scrollIntoViewIfNeeded(elem); } },
           })
         ),
         css.docRowWrapper.cls('-renaming', isRenaming),