gts check validates built JS files

Looks like compiled files in `build` directory are included in the check (by default). This is annoying to use on developer workstation because one has to remember to always do `npm run clean` before `npm run check`. A workaround is to add `ignorePatterns` in `.eslintrc.json`.

Steps to reproduce:

```bash
$ mkdir gtstest
$ cd gtstest
$ cat > package.json
{
"name": "gtstest",
"devDependencies": {
"gts": "^2.0.0"
},
"engines": {
"node": "10"
},
"private": true
}

$ npm i
$ npm run compile
$ npm run check

gtstest/build/src/index.js
1:1 error Replace `"use·strict"` with `'use·strict'` prettier/prettier
1:1 warning Strings must use singlequote quotes
2:32 warning Strings must use singlequote quotes
2:32 error Replace `"__esModule",·{·value:·true·` with `'__esModule',·{value:·true` prettier/prettier
4:7 error 'longString' is assigned a value but never used no-unused-vars
4:19 error Insert `⏎·` prettier/prettier
5:7 error 'trailing' is assigned a value but never used no-unused-vars
6:7 error 'why' is assigned a value but never used no-unused-vars
8:3 error Delete `··` prettier/prettier
9:1 error Delete `··` prettier/prettier
10:5 error Delete `····` prettier/prettier
```