Nunjucks v4.0.0-alpha.3 FULL ASYNC

• Full conversion of internals to use async/await. This is a big win as the sync/async boundaries in Nunjucks were a huge point of confusion for folks. Adds support for asynchronous content in: macros, loops ({% for %} without asyncEach), {% set %} (without setAsync). Looking forward to getting rid of these warnings on the docs:
  • https://www.11ty.dev/docs/languages/nunjucks/#warning-macros-are-not-async-friendly
  • https://www.11ty.dev/docs/languages/nunjucks/#warning-set-is-not-async-friendly
• Better fix for no-argument unpaired shortcode parse bug. Fixes mozilla#158 (Related to 11ty/buildawesome#3788 11ty/buildawesome#372)

Full Changelog: v4.0.0-alpha.2...v4.0.0-alpha.3