Fix issues blocking usage with modern Ruby (3.0+)

[transplier]

At some point in the last few years (likely around Ruby 3.2.0), this project stopped working on modern ruby:

• The gem explicitly refused to install under Ruby 3 (spec.required_ruby_version = '~> 2').
• Ruby 3.2.0 removed File.exists?, causing most functionality to break.
• Bypassing the gem version check, the last version that could theoretically run this software was 3.1.0, which went out of support earlier this year.

To support our usage, I went ahead and:

• Switched to File.exist?
• Updated the main gems (notable exception: haml 6 removed helpers the documentation site depends on, and I wasn't able to find any migration path that preserved formatting - so I just went to haml 5).
• Modernized gem setup.
• Replaced long-deprecated sass gem with sassc.
• Set minimum ruby version to the oldest still-supported ruby version (3.2).
• Set .ruby-version to latest release (3.4.7) (and added .tool-version for folks using asdf etc)

My partner and I just started our Twee2 journey, so pardon any oversights.

I tested (on Kubuntu 24.04.03 LTS):

• Generation of "Escape From Earth" sample story from the documentation.
• Reversing the generated html.
• Building the public website (not deployed, obviously).

I also noticed that the results of rake web:build are committed into the repo. Would you like me to check in the generated assets as well?

[transplier]

Should address:

• My ruby is too new for twee2? #64
• Vulnerability in HAML 4 #52
• Ruby Sass will become unmaintained in March 2019 #50