Introduce a minify-html plugin

[BlindDespair]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/scullyio/scully/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

@SanderElias I suggest to include MinifyHtml plugin as official @scullyio plugin since it is a useful plugin for the Scully ecosystem but seems to be no longer maintained while having certain issues.

I submit a PR that includes the plugin source code, docs for using it and build configs.
Besides that I updated it to use postProcessByHtml plugin type instead of deprecated render so Scully no longer gives a warning when running it with this plugin. (Related to original issue)
Also I fixed the bug where this plugin was messing with the transfer state script, so adding {minifyJS: true} is no longer required for Scully to work properly with this plugin.

I tested it locally with one of my projects and it worked with a small note, that when I used it via npm link Scully complained that plugin minifyHtml was not found, so I had to add registerPlugin manually in my project's config, but I believe it's due to how npm link works, plugin was registering itself in different instance of scully installation than what my project used.

[SanderElias]

@BlindDespair, sorry that this is taking some time, but there are a couple of things slightly off, and that is our fault.
We are currently working on a schematic to add a new plugin to our repo, so that becomes much easier.
I'll ping you when we have that done. It's going to be a small refactor of your work, but in its current form, we can't merge it.

[BlindDespair]

@SanderElias it's not a problem. I will fix it once your schematic is out.

[SanderElias]

@BlindDespair, Ok, I will close this PR for now. and let you know when we are ready for this.