gar-yaml: load yaml interpolating env variables

Example

DENO_ENV: ${{DENO_ENV}}

import { loadYaml } from 'https://deno.land/x/garn_yaml@v0.1.2/mod.ts';
import { assertEquals } from 'https://deno.land/std@0.79.0/testing/asserts.ts';

const yamlObject = await loadYaml('./test.yml');
assertEquals(typeof yamlObject, 'object');
assertEquals(yamlObject.DENO_ENV, 'development');