feat: Add @typescript-eslint/explicit-module-boundary-types

BREAKING CHANGE: Exported functions now need to specify an explicit return type. This enforces everyone to be explicit about the public interface of the module. This is likely to increase the developer experience because type errors will show up closer to the actual error.

typescript.js

@@ -76,6 +76,12 @@ module.exports = {
                         },
                     },
                 ], // https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/explicit-member-accessibility.md
+                "@typescript-eslint/explicit-module-boundary-types": ["error", {
+                    allowDirectConstAssertionInArrowFunctions: true,
+                    allowHigherOrderFunctions: true,
+                    allowTypedFunctionExpressions: true,
+                    allowedNames: [],
+                }], // https://github.com/typescript-eslint/typescript-eslint/blob/v2.19.0/packages/eslint-plugin/docs/rules/explicit-module-boundary-types.md
                 "@typescript-eslint/func-call-spacing": ["error"], // https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/func-call-spacing.md
                 "@typescript-eslint/generic-type-naming": "off", // https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/generic-type-naming.md
                 "@typescript-eslint/indent": ["error", 4, options["indent"]], // https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/indent.md