Add observer gem to gemspec

[marcoroth]

Type of PR

Enhancement

Description

Ruby 3.4 removes the observer gem from the standard gems which is why we have the declare the dependency explicitly now.

From https://www.ruby-lang.org/en/news/2023/12/25/ruby-3-3-0-released/:

Standard library updates

RubyGems and Bundler warn if users do require the following gems without adding them to Gemfile or gemspec. This is because they will become the bundled gems in the future version of Ruby.

Fixes #293

Why should this be added

To provide Ruby 3.4+ support.

Checklist

• My code follows the style guidelines of this project
• Checks (StandardRB & Prettier-Standard) are passing

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