(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/TreeModel.ts

@@ -207,7 +207,7 @@ export class TreeNodeRecord implements TreeNode {
   // Get all the records included in this item.
   public getRecords(): TreeRecord[] {
     const records = [] as TreeRecord[];
-    if (this.index !== "root") {records.push(this._records[this.index]); }
+    if (this.index !== "root") { records.push(this._records[this.index]); }
     walkTree(this, (item: TreeItemRecord) => records.push(this._records[item.index]));
     return records;
   }
@@ -255,7 +255,7 @@ export function find(model: TreeNode, func: (item: TreeItem) => boolean): TreeIt
   if (children) {
     for (const child of children.get()) {
       const found = func(child) && child || find(child, func);
-      if (found) {return found; }
+      if (found) { return found; }
     }
   }
 }