feat(linter): add codespell linter

[ideaship]

Add the codespell linter which uses a dictionary of common misspellings and ignores unknown words.

This is an updated version of #4143 which was closed as stale and could not be reopened by the maintainer.

Fixes #6255

Proposed Changes

Add the codespell linter which uses a dictionary of common misspellings and ignores unknown words.

Readiness Checklist

Author/Contributor

• Add entry to the CHANGELOG listing the change and linking to the corresponding issue (if appropriate)
• If documentation is needed for this change, has that been included in this pull request

Reviewing Maintainer

• Label as breaking if this is a large fundamental change
• Label as either automation, bug, documentation, enhancement, infrastructure, or performance

---

Note

Introduce the codespell linter and integrate it across core, flavors, Docker images, tests, docs, and configuration.

• New linter: codespell
  • Add descriptor and config in megalinter/descriptors/spell.megalinter-descriptor.yml with .codespellrc support.
  • Integrate into flavors and matrices: update flavors/*/flavor.json, megalinter/descriptors/all_flavors.json, and schemas.
  • Install in images: add pip arg, venv, and PATH in root Dockerfile and all flavor Dockerfiles; add standalone image linters/spell_codespell/Dockerfile.
  • CI/Tests: add unit test spell_codespell_test.py and fixtures under .automation/test/spell_codespell/.
  • Docs/Changelog: update docs/standalone-linters.md and CHANGELOG.md.
  • Generated metadata: update linter-helps.json, linter-links-previews.json, linter-versions.json, linters_matrix.json.
  • Repo config: add words to .cspell.json; disable SPELL_CODESPELL errors in .mega-linter.yml.

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