(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/components/CellPosition.ts

@@ -28,8 +28,9 @@ export function samePosition(a: CellPosition, b: CellPosition) {
  * @param docModel Document model
  */
 export function fromCursor(position: CursorPos, docModel: DocModel): CellPosition | null {
-  if (!position.sectionId || !position.rowId || position.fieldIndex == null)
+  if (!position.sectionId || !position.rowId || position.fieldIndex == null) {
     return null;
+  }
 
   const section = docModel.viewSections.getRowModel(position.sectionId);
   const colRef = section.viewFields().peek()[position.fieldIndex]?.colRef.peek();