feat: add emacs

[peccu]

Fixes #577

[amiorin]

We install emacs through homebrew and doomemacs with a private feature.

[evilhamsterman]

This project is not associated with the devcontainer project and appears to have been abandoned #628 . The official project has removed all devcontainer-contrib features from their listing. devcontainers/devcontainers.github.io#451. There is a fork that is being worked on to update and maintain the features https://github.com/devcontainers-extra/features