refactor(pwa): simplify registerSW code, fix ESLint errors

[Josh-Cena]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

Before we aggressively bump our browserslist to only support modern browsers, or before IE11 dies out from the default browserslist inclusion, we must not use async-await in the client bundle to reduce bundle size.

Floating promises are a problem if async-await is not used; however, this can be greatly remedied by using TS-ESLint's no-floating-promises rule. We are still not ready to enable type-aware lint rules in CI, but I'm already working with it locally.

As a side-fix, I've fixed some new ESLint errors as they emerged after the recent code changes.

Test Plan

Test links

Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/

Related issues/PRs

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 86	🟢 100	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 86	🟢 100	🟢 100	🟢 100	🟢 90	Report

[github-actions]

Size Change: -32 B (0%)

Total Size: 801 kB

Filename	Size	Change
website/build/assets/js/main.********.js	603 kB	-32 B (0%)

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	52.6 kB
website/build/assets/css/styles.********.css	106 kB
website/build/index.html	38.9 kB

_{compressed-size-action}

[Josh-Cena]

😨 Why only -62B... That seems like optimization elsewhere... I thought it would at least remove some regeneratorRuntime calls...

[Josh-Cena]

OK, it seems avoiding async-await does save bundle size, but very marginal, because minification is sufficient to reduce all the generator calls to a few bytes. Takeaway is don't try to take the job of the minifier.

Still going to merge it because of the ~70 line reduction in length.