chore: add ESLint and Prettier

[stephenmathieson]

This patch introduces ESLint and styles all code with Prettier. I've configured Prettier to match the existing code-style as much as possible.

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• [X] Follows the commit message policy, appropriate for next version
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• [X] Includes new tests, or was unnecessary
• [X] Code is reviewed for security by: << Marcy Sutton >>

[marcysutton]

Actually it looks like there's a failing test due to some new formatting:

AssertionError: expected '
function(callback) {\n\t\t\t\t\tvar script = document.createElement(\'script\');\n\t\t\t\t\tscript
.innerHTML =\n\t\t\t\t\t\t\'document.documentElement.classList.add("deque-axe-is-ready");\';\n\t\t\t\t\tdocument.documentElement.appendChild(script);\n\t\t\t\t\tcallback();\n\t\t\t\t}

' to include '
.innerHTML = \'document.documentElement.classList.add("deque-axe-is-ready");\''

That looks sorta brittle....perhaps we should refactor it in a separate PR?

[stephenmathieson]

Wow, that is a strange test. Yes, I'll open another PR updating it. Thanks for pointing it out.

Opened PR #74.