(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/plugin/grist-plugin-api.ts

@@ -172,7 +172,7 @@ if (typeof window !== 'undefined') {
   // running under mocha. For now, we only provide a disfunctional implementation. It allows
   // plugins to call methods like registerFunction() without failing, so that plugin code may be
   // imported, but the methods don't do anything useful.
-  rpc.setSendMessage((data) => {return; });
+  rpc.setSendMessage((data) => { return; });
 }
 
 function createRpcLogger(): IRpcLogger {