(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/editableLabel.ts

@@ -105,7 +105,7 @@ export function rawTextInput(value: Observable<string>, save: SaveFunc, onChange
   let inputEl: HTMLInputElement;
 
   // When label changes updates the input, unless in the middle of editing.
-  const lis = value.addListener((val) => { if (status !== Status.EDITING) { setValue(val); }});
+  const lis = value.addListener((val) => { if (status !== Status.EDITING) { setValue(val); } });
 
   function setValue(val: string) {
     inputEl.value = val;