Limit files to be included in the package

[ocean90]

See https://docs.npmjs.com/cli/v7/configuring-npm/package-json#files

Before

❯ npm pack
npm notice
npm notice 📦  @midzer/tobii@2.0.7
npm notice === Tarball Contents ===
npm notice 8.2kB  .eslintrc
npm notice 1.4kB  .stylelintrc
npm notice 4.8kB  dist/tobii.min.css
npm notice 672B   demo/index.html
npm notice 688B   demo/modern.html
npm notice 173B   src/js/browser.js
npm notice 553B   src/js/polyfill/CustomEvent.js
npm notice 42.2kB src/js/index.js
npm notice 17.3kB dist/tobii.js
npm notice 18.5kB dist/tobii.min.js
npm notice 17.6kB dist/tobii.modern.js
npm notice 17.4kB dist/tobii.module.js
npm notice 17.5kB dist/tobii.umd.js
npm notice 1.8kB  package.json
npm notice 1.1kB  LICENSE.md
npm notice 10.1kB README.md
npm notice 87B    src/scss/_functions.scss
npm notice 590B   src/scss/_variables.scss
npm notice 5.3kB  src/scss/tobii.scss
npm notice 28.5kB midzer-tobii-2.0.7.tgz
npm notice 640B   .github/FUNDING.yml
npm notice 1.1kB  .github/workflows/npm-publish.yml
npm notice === Tarball Details ===
npm notice name:          @midzer/tobii
npm notice version:       2.0.7
npm notice filename:      midzer-tobii-2.0.7.tgz
npm notice package size:  61.6 kB
npm notice unpacked size: 196.2 kB
npm notice shasum:        9dd4bf39bbbfa7c9b235b8739d67707054f13848
npm notice integrity:     sha512-SC9w6USHD/yXN[...]PavjA8U0I8nMg==
npm notice total files:   22
npm notice
midzer-tobii-2.0.7.tgz

After

❯ npm pack
npm notice
npm notice 📦  @midzer/tobii@2.0.7
npm notice === Tarball Contents ===
npm notice 4.8kB  dist/tobii.min.css
npm notice 173B   src/js/browser.js
npm notice 553B   src/js/polyfill/CustomEvent.js
npm notice 42.2kB src/js/index.js
npm notice 17.3kB dist/tobii.js
npm notice 18.5kB dist/tobii.min.js
npm notice 17.6kB dist/tobii.modern.js
npm notice 17.4kB dist/tobii.module.js
npm notice 17.5kB dist/tobii.umd.js
npm notice 1.8kB  package.json
npm notice 1.1kB  LICENSE.md
npm notice 10.1kB README.md
npm notice 87B    src/scss/_functions.scss
npm notice 590B   src/scss/_variables.scss
npm notice 5.3kB  src/scss/tobii.scss
npm notice === Tarball Details ===
npm notice name:          @midzer/tobii
npm notice version:       2.0.7
npm notice filename:      midzer-tobii-2.0.7.tgz
npm notice package size:  28.5 kB
npm notice unpacked size: 155.0 kB
npm notice shasum:        dab4506b759d1d6db7bac3b8dffe4993c0ca5f92
npm notice integrity:     sha512-pPa3+zIBF5Kt0[...]q281BxO55YuFg==
npm notice total files:   15
npm notice
midzer-tobii-2.0.7.tgz

[midzer]

Awesome!
I've never heard of files until now...

[midzer]

Thanks for you working on tobii @ocean90

Once #54 is confirmed, I'll do a new release.