Improve DX, add failing a11y tests

[mark-wiemer]

(DX == developer experience)

• Add hugo-bin to package.json depdendencies so users don't have to manually install an arbitrary global version
• Document existing spellcheck tests
• Add sort-package-json to help keep package.json in a consistent order (not enforced yet, considering that out of scope for this PR)
• Add failing a11y tests that run on the locally-built version of the site. Fixing the tests will be for a second PR. Ref Standard accessibility checks #72

Notes

• If this PR is accepted, then we should open an issue to update hugo.yaml to use the package.json version of hugo-extended instead of manually fetching that specific version. However, I'm not sure how to test all this, and was getting some CSS issues when I ran the hugo --gc ... command in hugo.yaml locally. For now, considering that change out of scope for this PR. Ref Use consistent Hugo version for CI and CD #144
• Note hugo-bin, the upstream of hugo-extended, is not the "extended" build of Hugo and thus fails to build the site. hugo-extended it is :)

[wsor4035]

note that "a future pr" should add git submodule stuff into the npm scripts

[wsor4035]

seems good enough. still supports just plain hugo for base work so im happy