(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/server/lib/PluginManager.ts

@@ -51,7 +51,7 @@ export class PluginManager {
     };
   }
 
-  public dirs(): PluginDirectories {return this._dirs; }
+  public dirs(): PluginDirectories { return this._dirs; }
 
   /**
    * Create tmp dir and load plugins.