chore: Replace tslint with eslint

- Remove tslint and its configurations
- Add eslint and set default configurations

Palantir [announced][1] that it will deprecate TSLint in favour of
typescript-eslint.

The TypeScript team [announced][2] that themselves will start using
typescript-eslint instead of tslint.

[1]: https://medium.com/palantir/tslint-in-2019-1a144c2317a9
[2]: microsoft/TypeScript#30553

.eslintrc.js

@@ -0,0 +1,26 @@
+module.exports = {
+  env: {
+    browser: true,
+    es6: true,
+    node: true,
+  },
+  extends: [
+    'eslint:recommended',
+    'plugin:react/recommended',
+    'plugin:@typescript-eslint/eslint-recommended',
+    'plugin:@typescript-eslint/recommended',
+    'plugin:@typescript-eslint/recommended-requiring-type-checking',
+    'prettier',
+    'prettier/react',
+    'prettier/@typescript-eslint',
+  ],
+  globals: {
+    Atomics: 'readonly',
+    SharedArrayBuffer: 'readonly',
+  },
+  parser: '@typescript-eslint/parser',
+  parserOptions: {
+    project: './tsconfig.json',
+  },
+  rules: {},
+};

gatsby-config.js

@@ -15,8 +15,18 @@ module.exports = {
     },
     `gatsby-transformer-sharp`,
     `gatsby-plugin-sharp`,
-    `gatsby-plugin-typescript`, 
-    `gatsby-plugin-tslint`,
+    `gatsby-plugin-typescript`,
+    {
+      resolve: `gatsby-plugin-eslint`,
+      options: {
+        test: /\.(js|ts)x?$/,
+        exclude: /(node_modules|.cache|public)/,
+        options: {
+          emitWarning: true,
+          failOnError: false,
+        },
+      },
+    },
     {
       resolve: `gatsby-plugin-manifest`,
       options: {
@@ -33,4 +43,4 @@ module.exports = {
     // To learn more, visit: https://gatsby.dev/offline
     // `gatsby-plugin-offline`,
   ],
-}
+};