Add Stoplight Spectral linter for yaml

[t2h5]

• Add Stoplight Spectral linter for yaml and json.
  • https://github.com/stoplightio/spectral
  • It helps to edit OpenAPI Schema.

[hsanson]

This linter is very useful, did not know about it. Would be great if this PR also added support for JSON files. It should not be difficult to achieve.

Check for example redpen linter that supports many file types: markdown, rst, tex, text, etc. All methods used by the linter are defined in autoload/ale/handlers/redpen.vim and then only the linter definitions are in the respective file type paths: ale_linters/text/redpen.vim, ale_linters/markdown/redpen.vim, etc.

[t2h5]

Added JSON support.

[hsanson]

@w0rp seems travis CI is somehow stuck... may PRs are waiting for status report and cannot be merged.

[hsanson]

Changes are good and tested the linter works properly on my own openapi yaml and json definitions.

[hsanson]

ALE migrated from Travis to Github Actions (see #3548) so it is necessary to rebase this PR from latest master branch to trigger the required checks.

# Add dense-analysis as remote upstream on your local fork. Not 
# needed if already added.
git remote add upstream https://github.com/dense-analysis/ale.git

# Sync your local master with upstream master
git checkout master
git fetch upstream master
git merge upstream/master

# Rebase the PR branch with master
git checkout my-branch-name
git rebase master   # Fix any conflicts you may have

# Force push the updated PR branch to origin to trigger the checks
git push -f origin my-branch-name

[hsanson]

Good to go, thanks.