chore: Migrate to Melos 8 with a pub workspace and add the Flame release workflows

[spydon]

Description

Migrates the monorepo to the latest Melos (8.x) on top of a pub workspace and sets up the same
tooling and automation as the flame repository.

Melos and pub workspace

• The root pubspec.yaml is now a pub workspace that lists all packages, and the Melos
  configuration lives in its melos section, so melos.yaml is gone.
• Every package declares resolution: workspace and the SDK constraints are aligned to
  Dart >=3.13.0 / Flutter >=3.47.0, which is what the workspace resolution requires
  (very_good_analysis 11 needs Dart 3.13).
• The scripts mirror flame's: format-check, lint:all, test:select, coverage, upgrade,
  markdown-check / markdown-fix, plus editor-run / editor-build for the editor app.
• Per package pubspec.lock files, IDE files and the leftover .github directory inside
  mini_sprite_editor are removed and ignored through the same .gitignore as flame.

Workflows

• cicd.yml: format, analyze on the lowest supported Flutter version, analyze on latest stable,
  markdown lint and tests, replacing the seven per package workflows.
• release-prepare.yml, release-tag.yml and release-publish.yml: the flame release flow.
  Prepare release runs melos version and opens a release PR, merging it tags the packages and
  triggers the publish workflow, which publishes to pub.dev with OIDC and creates GitHub releases.
• title-validation.yml and spell_checker.yml with a cspell configuration and dictionaries.
• gh-pages.yml now uses bluefireteam/flutter-gh-pages to deploy the editor.

Repository files

• Adds CONTRIBUTING.md, FUNDING.yml, the flame pull request template and the markdownlint
  configuration.
• Runs dart format on all packages with the current formatter, fixes the markdown lint findings
  in the READMEs, and fixes the typos that cspell found (including the toogleGrid and
  newPropery localization keys in the editor).
• Applies dart fix for the lints that very_good_analysis 11 enables on Dart 3.13, which
  rewrites constructors to the new const new(...) syntax in mini_sprite, flutter_mini_sprite
  and mini_treasure_quest. Say so if you would rather disable unnecessary_type_name_in_constructor
  instead.

Locally, melos bootstrap, melos run format-check, melos exec -- dart analyze --fatal-infos .,
melos run test:select --no-select, markdownlint and both cspell scripts all pass.

Notes for the maintainers

• Publishing needs the packages on pub.dev to have automated publishing from GitHub Actions
  enabled for this repository, with the release-publish.yml workflow and the pkg-v* tag
  pattern.
• The gh-pages workflow now also runs on every push to main, like in flame. Drop the push
  trigger if the editor should only be deployed manually.

Checklist

• I have followed the Contributor Guide when preparing my PR.
• [-] I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation and added dartdoc comments with ///.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.