Adopt eslint-plugin-unicorn alongside the rest of the sitespeed.io stack

[soulgalore]

Brings coach-core onto eslint-plugin-unicorn 64 — the same major
sitespeed.io is on — and migrates the eslint config from .cjs to .js
(ESM), which is what the plugin requires and what coach-core's already
ESM package implies anyway. The unicorn rule set surfaced 354 findings,
276 of them auto-fixable: a sweep through the codebase toward modern
array methods (for…of over .forEach, flatMap over map().flat(),
some over findIndex truthy checks), node-native helpers
(Number.parseInt, String.fromCodePoint, structuredClone, String#slice,
String#replaceAll, String.raw, includes), and tidier conditionals
(explicit length checks, no nested ternaries, no useless undefined).

The remaining hand-fixes are small. The wappalyzer engine moves its
priority arrow out of resolve() into a module-level helper; an
exec()-as-truthy check becomes a .test(); lib/index.js hoists
getFolders to module scope and stops chaining off an awaited stat()
call. A single inline disable lives in lib/dom/bestpractice/charset.js
because the page's document.characterSet returns the literal 'UTF-8'
and the unicorn rule would rewrite it to 'utf8' and silently break
detection.

The config disables the same six unicorn rules sitespeed.io disables,
plus three more that don't fit coach-core's situation: no-null
(the vendored wappalyzer engine deliberately uses null to match the
upstream data format), import-style (destructured node:* imports are
clearer than forcing default imports), and prefer-module scoped to
lib/dom/** (those files are not modules — they are IIFE fragments
that get concatenated into dist/coach.min.js and evaluated in the
browser via WebDriver executeScript).

Co-authored-by: Claude noreply@anthropic.com