ci: Add CI/CD

[Riernar]

Requested feature

Add a CI and a CD:

• CI should:
  • compile the sheet and run tests (if applicable) in PRs and main branch
  • Verify that commit message conform to expected format for the main branch. For PR, check the PR title
  • Verify that github issue link systematically include the Org/repo prefix, to avoid mixup when merging in Roll20/roll20-character-sheets
• CD should trigger on new github release and open a PR into the root Roll20/roll20-character-sheets with a release build
  • build the sheet in release mode, with appropriate asset prefix in SCSS
  • push this build to https://github.com/Riernar/roll20-character-sheets
  • Open the PR between the created branch and https://github.com/Roll20/roll20-character-sheets

Alternative solutions

Manual release. Those are error-prone and painful due to the use of git subtrees.

Additional context

• Might require a deploy token as secret for write access to https://github.com/Riernar/roll20-character-sheets
• Might require the gh command line

[Riernar]

Also track when https://github.com/Roll20/roll20-character-sheets/arm5-sheet changes and open local PR, so that we do not miss updates from the upstream. If there are updates, open a PR here maybe?