(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/common/UserAPI.ts

@@ -633,8 +633,8 @@ export class DocWorkerAPIImpl extends BaseAPI implements DocWorkerAPI {
     super(_options);
   }
 
-  public async importDocToWorkspace(uploadId: number, workspaceId: number, browserSettings?: BrowserSettings)
-    : Promise<DocCreationInfo> {
+  public async importDocToWorkspace(uploadId: number, workspaceId: number, browserSettings?: BrowserSettings):
+      Promise<DocCreationInfo> {
     return this.requestJson(`${this.url}/api/workspaces/${workspaceId}/import`, {
       method: 'POST',
       body: JSON.stringify({ uploadId, browserSettings })