Error on unused eslint directives, reenable lost eslint rules

[jakebailey]

Passes --report-unused-disable-directives to eslint, which complains when a directive is unused. This is poised to be a warning anyway in eslint v9.

Also, reenable unified-signatures and no-unused-expressions, which were mistakenly forgotten when we revamped our eslint config to instead make use of recommended presets.

[Mateusnasciment]

Enable eslint rules, prepare for v9, report unused directives hehehehe