fix: cli trying to transform directories

[Esemesek]

Motivation

Running linaria CLI with a glob wildcard produced an error if the directory contains nested folders.

 /build
    /components
      /SomeComponent.js
    /index.js
 /src
    /components
      /SomeComponent.tsx
    /index.ts

yarn linaria -o build -r src -i build ./src/**/*

Summary

Simply check if the file is a directory before we run any transformations.

Test plan

It works with nested directories - scenario presented above. There is no unit testing set up for @linaria/cli so I skipped adding test cases for this.

[Anber]

Thank you @Esemesek!