DEV: Modernize codebase

[keegangeorge]

This theme-component was recently moved from my personal profile (/keegangeorge/discourse-copy-post) to the Discourse org profile. Along with that move, we want to make some modernizations to prepare this theme-component to become #official. This PR handles those modernizations.

In particular we:

• update linting, configs, workflows, etc.
• remove widget in favour of Glimmer .gjs component
• update API usage to use valueTransformer instead of decorateWidget to add the post menu button
• make improvements to behaviour by way of introducing new settings:
  • choosing between Markdown / HTML for the copied post
  • using allowed_groups setting instead of trust level setting
• remove acceptance test in favour of system spec