Update eslint to 7.0, add babel-eslint plugin.

Add babel-eslint plugin so that eslint can recognize optional chaining
syntax (a?.b).

.eslintrc.yml

@@ -8,9 +8,13 @@ parserOptions:
 globals:
   $: false
   require: false
+parser: babel-eslint
+plugins:
+  - babel
 rules:
   # https://eslint.org/docs/rules/
   array-callback-return: error
+  babel/no-unused-expressions: error
   block-scoped-var: error
   consistent-return: error
   default-case: error
@@ -32,7 +36,7 @@ rules:
   no-sequences: error
   no-shadow: error
   no-shadow-restricted-names: error
-  no-unused-expressions: error
+  #no-unused-expressions: error
   no-unused-vars:
     - error
     - argsIgnorePattern: "^_"