(core) Configure more comprehensive eslint rules for Typescript

Summary:
- Update rules to be more like we've had with tslint
- Switch tsserver plugin to eslint (tsserver makes for a much faster way to lint in editors)
- Apply suggested auto-fixes
- Fix all lint errors and warnings in core/, app/, test/

Test Plan: Some behavior may change subtly (e.g. added missing awaits), relying on existing tests to catch problems.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2785

stubs/app/client/ui/CustomThemes.ts

@@ -4,7 +4,7 @@ export type ProductFlavor = string;
 export interface CustomTheme {
   bodyClassName?: string;
   wideLogo?: boolean;
-};
+}
 
 export function getTheme(flavor: string): CustomTheme {
   return {