Remove demanding dependency fast_yaml

[almirsarajcic]

There were some complaints that :fast_yaml couldn't compile for some users. I've investigated the issue, but couldn't find an alternative as there's no native YAML encoder in Elixir/Erlang. Now that we have native JSON implementation, who knows...

Anyway, I didn't spend much time on that until I got the same issue. Of course, this time I've had to find a solution. So, after again wasting a couple of hours trying to set up :fast_yaml dependency and searching for an alternative, I finally decided to write my own YAML encoder. I didn't need to support everything YAML has to offer. I guess that laziness was also one of the reasons behind GitHub's insufficient support of the YAML standard which forced me to create this generator in the first place 😁

I'm not proud of this code, but at least it works™. And it's closer to GitHub's examples than the version that uses :fast_yaml.

Also, this fixes #2.
Now it's possible to have pull_request: instead of pull_request: [].
Look at:

github_workflows_generator/.github/workflows/ci.yml

Lines 3 to 7 in b48263b

	on:
	pull_request:
	push:
	branches:
	- main

Linter didn't complain about the newly generated ci.yml file and Prettier didn't find anything to change.