Split out TypeScript configuration to separate file.

In #10, the typescript-eslint plugin and its recommended ruleset was
added, but this inadvertently also covered .js files, and caused
problems when used with codebases containing mixed JS/TS files.

To avoid this issue, we define another extendable ruleset
@glints/eslint-config/typescript. Projects that want to apply this
ruleset to just TypeScript files can define "overrides": ["**/*.ts",
...].

index.js

@@ -1,11 +1,6 @@
 module.exports = {
-  parser: '@typescript-eslint/parser',
-  plugins: [
-    '@typescript-eslint',
-  ],
   extends: [
     'eslint:recommended',
-    'plugin:@typescript-eslint/recommended',
   ],
   rules: {
     'strict': [2, 'never'],

typescript.js

@@ -0,0 +1,5 @@
+module.exports = {
+  parser: '@typescript-eslint/parser',
+  plugins: ['@typescript-eslint'],
+  extends: ['plugin:@typescript-eslint/recommended'],
+};