Update to ESLint 10 and align with sitespeed.io's ESLint stack

[soulgalore]

Brings coach-core onto the same ESLint 10 baseline as sitespeed.io and
browsertime: eslint 9.17 → 10.4, eslint-config-prettier 9.1 → 10.1.8,
eslint-plugin-prettier 5.2 → 5.5.5. @eslint/js and globals were silently
transitive under v9 and are now declared explicitly, since the flat
config imports them directly.

While there, the config dropped FlatCompat in favour of the flat-config
shape ESLint 10 expects natively — js.configs.recommended is referenced
directly, which lets the @eslint/eslintrc dependency go.

ESLint 10 ships two new eslint:recommended rules (no-useless-assignment
and preserve-caught-error) that surfaced six findings, all fixed in
place rather than disabled. Five were let foo = '' initializers where
every subsequent code path reassigns before reading; they become
let foo;. The sixth was a rethrow in the wappalyzer engine that lost
the original error — adding { cause: error } preserves the chain.

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